Implementation of relativistic species (photons, neutrinos) into
cosmology classes.

Add neutrinos and photons to cosmology classes using nuetrino
treatement from Komatsu et al. 2011 ApJS 180, 330 (the WMAP7
cosmology paper).

* Add parameters Tcmb0, Neff, and m_nu to all classes, with defaults.
* Add nu_relative_density fucntion to compute rho_nu / rho_gamma
* Update all efunc/inv_efunc calculations to include relativistic species
* Because specifying a flat cosmology is slightly tricker when there is
  a small non-zero Omega_rel, added a few Flat cosmologies to make things
  easier on the user (FlatLambdaCDM, FlatwCDM).
* Updated tests.
* Updated docs.

The new code is not tested in detail -- but the old tests have been
modified to work (since they all assumed Orel = 0.0).

The neutrino treatement is exact but assumes that all neutrino species
have the same mass.  For some high masses they aren't properly relativistic,
but in practice they will be so it seems convenient to include them
in Omega_relativistic (note that I don't call it Omega_radiation for this
reason).

It isn't totally clear what the best way to include relativistic species is.
There are a number of possibilities.

* Ignore neutrino masses, assume the normal number of neutrinos, and
  work purely with Omega_rad.  This is the simplest, but is rather limiting
  because it makes totally vanilla assumptions.
* Allow for neutrinos with masses, but use Omega parameters as much
  as possible.  This requires: Omega_photon, Omega_nu (the usual
  Omega_nu h^2 = \sum m_nu / 94 eV), N_eff (the effective number
  of nuetrino species), N_nu (the actual number of neutrino species).
  Note that N_eff != N_nu (and they can't be easily calculated from each
  other, although they don't differ by much).  Also note that this,
  the usual way Omega_nu is written, is very confusing because it
  virtually -never- actually corresponds to the contribution of neutrinos
  to the total energy density.  So it looks like an Omega_nu, but actually
  isn't.  Even for low mass neutrinos, Omega_relativistic != Omega_photon
  + Omega_nu if you write it this way, which is really confusing.
* Try to use observable parameters as much as possible;  These are
  Tcmb0, m_nu, and N_eff.  The advantage is that the paramaterization
  has 1) fewer parameters than the previous one because it relates
  better to the actual physics, and 2) it is more directly related to
  observables.  However, the parameterization may be a little less familiar.

The latter is used in this branch right now because it is simpler.
Note that all the parameters have reasonable defaults -- in fact, most
users never really need know they exist at all.
Removed massive nuetrinos, now only support massless ones.

* Parameters remain the same except that m_nu is removed
* Internal functions having to do with nuetrino masses removed
* Test cases added:
   ** A comparison with Ned Wright's Advanced Cosmological Calculator.
      Unfortunately, the NWACC isn't that accurate (and uses the
      wrong value of a_B), so the precision of this test is limited.
  ** A comparison with a numerical calculation using Mathematica,
      which is significantly more precise.
  ** A test against the special case of only Or and Om, which can
     be evaluated analytically for comparision.
* Documentation updated.

The code passes all tests having to do with the cosmology module,
and sphinx builds without complaint.
Minor doc fix for FLRW.Onu
Doc fix for FLRW.

Say that all neutrinos are massless, not all neutrinos have the same
mass.
Fix a few documentation typos.  Fix sphinx type spec.

Fix misspelling of neutrino (nuetrino) in a few places.
Remove () from sphinx documentation variable specification.
  Example:  z: (float) is now z: float.
Added a small amount of documentation of input/output variables
  to some minor FLRW methods that didn't have any.
Fix missing space around : for sphinx docs.  Add Flatw0waCDM.

It turns out sphinx binds the documentation incorrectly
if there is not a space on both sides of : in things like
z : ndarray.  So, fixed.

Added trivial Flatw0waCDM cosmology class.
Merge pull request #365 from aconley/relativistic_species

Implementation of relativistic species (photons, massless neutrinos) into cosmology classes.