Documented Model class

Also, I improved the .Rd files for all the other classes.  It's a little
confusing as to which package version really is better: 2.14 (at home)
omits duplicates, but also function names!  So I did a manual merge and
will just leave these versions alone as much as I can.
Put rgl aspect ratio code in own function
Added citation information
Setup steel strain demo (but Train.Model buggy)

First code relating to a demo for the package.  Made a demo directory
and script.  Other changes include:
  Dataset.R:  Improved Plot2D function: you can now adjust the vertical
    scale, and it doesn't force you to clear the plot
  utils.R:  Made a "pause" function for the demo
  Covariance.R: Removed a useless line

Model training is not working for aniso2D, but it works quite well for
regular SE.  In particular, it always maximizes the sigma's (and the
ell's are correspondingly too large).  I've verified that the parameter
values from DEoptim have significantly higher LogML than the parameters
where it's getting stuck.  Maybe it's some kind of boundary issue?
Maybe try enlarging the Model's bounds, then shrinking them back down?
L-BFGS-B really shouldn't propose outside the bounds, so we should still
be OK...
Expand bounds during training to prevent clamping

The optimizer seems to be getting confused when parameter values get
clamped.  I think I'll have better results if I just let L-BFGS-B take
care of the boundaries.  Expand the Model's bounds ($lower and $upper)
during training, then bring them back once we're done.
Merge branch 'develop' into strainDemo
Fixed bug for Model$Train with bounds

Bug: certain Models being trained would get stuck at the boundaries.
Turns out, this was because 'optim' *ignores* the names on the 'lower'
and 'upper' vectors, so boundaries weren't necessarily matched with the
corresponding parameters.
Fix: caused getLower and getUpper to omit values for constant
parameters, in accordance with the behaviour of getParams.
Merge branch 'strainDemo' into develop
Bumped version number to 0.2-0
steelStrain demo now plots best-fit surface
Changed documentation to use R 2.15

Most .Rd files only changed the ordering of a few lines.
Fixed: had double-counted .Model in docs

I had written
@S3method getLower.Model Model
instead of
@S3method getLower Model
which is boneheaded.
Made documentation compatible with NAMESPACE

- Added export statements for print and clone method
- For now, I have to manually add the constructors to NAMESPACE, because
  they use R.oo instead of roxygen.
- Gave in to the Collate message in the DESCRIPTION file, which the
  newer devtools generates
Made public R.oo play nice with NAMESPACE

Apparently I need to export both the generic function and the fully
qualified name for each class, or else the R.oo syntactic sugar won't
work.  This commit doesn't represent a working state, because I need to
fix the private files, but it should be OK for the public ones.
Fixed oversights in documentation

- getVaryingParamNames: I had written roxygen comments, but I forgot to
  write "#'" instead of "#" so roxygen would actually see them
- setParams.Covariance had been missing export and S3method tags; these
  are now added
Fixed demo (removed syn-sugar for private methods)

R.oo provides syntactic-sugar field access: e.g., instead of
  Method(this=object, ...)
you can write
  object$Method(...)
which is a more "object oriented" way of doing things.  Unfortunately,
this seems to rely on the fully qualified method name, i.e.
  Method.class
being exported to the namespace.  Private methods should *not* be
exported to the namespace.  Hence, we can't use the "nice" way to access
private methods.
Use the "modern" approach for sd() on data.frame
steelStrain demo: show TRUE datapoints in gap
Removed redundant 'gppois:::' qualifiers

I was a little overzealous adding 'gppois:::' before each private
method.  This appears to be unnecessary for code that is WITHIN the
package.  (Code "outside" the package, such as the steelStrain.R demo,
still needs this qualifier.)
Setup structure for unit tests

Note: there is no reason to believe these tests will run, or that they
are reasonably complete.