#562 - Replaced GSParams alias_threshold with folding_threshold

As discussed on #562 , we have decided that the alias_threshold parameter in the GSParams has been confusingly named up to now. It only refers to "aliasing" in real space, i.e. folding/overlapping flux from neighbouring copies of the profile.

We decided it would be clearer to use folding_threshold, and then from now on to keep to the convention of using "aliasing" to describe the effects of DFT-related folding in the Fourier domain, while using folding to describe the same phenomenon in the real domain.  The `folding_threshold` name is the terminology we are currently planning to use in the GalSim paper.

Thanks to Mike's tip (https://github.com/GalSim-developers/GalSim/issues/562#issuecomment-45621866) I think I have successfully changed everything in the main body of the codebase, although I would be grateful for everyone trying a full reinstall (`scons -c`, `rm -f .scon*`, the works) and running the tests and example checking scripts to verify that.

There are two questions I do have, though:

* Should I make a (deprecated) `alias_threshold` be still available?  It's a slight extra pain since the GSParams are actually C++, but even in the course of writing this I've started to convince myself that I ought to do this... (Is there a way to make a nice warning message in C++?)

* What to do with the scripts in `devel/`?  It has not been our standard practice to keep these up to date with all changes to the API, and so following this precedent we would leave them as-is, opting to retain historical consistency in the devel scripts and not open ourselves to always having to maintain them (which does not seem popular).  I am happy with that, but I think we ought to discuss it.  The alternative would be that I modify the scripts of myself, @tomaszkacprzak , @msimet , @rmandelb etc. in `devel/`, but this might only bring them partially up to date anyway.  I think that updating them would be another issue, but I'm open to other opinions!
The only problem is, there isn't a Python layer for the GSParams... We use the Boost.Python wrapper directly.  It wouldn't be too hard to rename the Boost.Python object something like `_GSParams` and make a Python-layer `GSParams` container, of course.  Or might there be something we could do at the C++ layer?