Improve comment description of estimateCombinedCovariance method
Generalize parsing of distortion model strings to prepare for more general model
Add support for alternate distortion model config syntaxes
Add full set of distortion model syntax variations
Implement new rP,rT terms in distortion model and update header docs
Update INI file to use (rP,rT) binning by default
Add optional rP,rT terms to the polynomial parameterization of distortion models. The excerpt
below from the BroadbandModel header comments describes how the command-line syntax for
distortion models has been generalized to accommodate the new terms. Note that the original
syntax is still supported, for backwards compatibility, but should now be prefixed with "r,mu,z="
with consistency with the other formats.

(from BroadbandModel.h)
Creates a broadband model with parameters named "<tag> z%d mu%d r%d rP%d rT%d" where each
%d slot is filled with a range of indices specified in paramSpec using python array notation:
  n => only index is n
  n1:n2 => indices n1...n2
  n1:n2:dn => indices n1...n2 in steps of dn
The set of axes that will be used is determined by one of the following prefixes:
  r,mu=
  rP,rT=
  r,mu,z=
  rP,rT,z=
  r,mu,rP,rT,z=
For example, the string "r,mu=-2:0,0:4:2" specifies a polynomial in (r/r0)^n P(ell,mu) with
n=-2,-1,0 and ell = 0,2,4. If the prefix is missing and 3 axes are specified, then
the r,mu,z syntax is assumed by default, for backwards compatibility. The parameters r0,z0
are used to normalize the polynomial expansion factors as follows:

  (r/r0 - q)^n, (rP/r0 - q)^n, (rT/r0 - q)^n, ((1+z)/(1+z0))^n

where q = 0 if n <= 0 or else q = +1, e.g. ((r-r0)/r0)^2.