#426 Improve behavior of Sersic class

There are a number of improvements to the Sersic class on this branch triggered by David noticing some ripples in images that were higher amplitude than they should have been.  But then I pulled in some other improvements from other branches in order to get them onto master sooner, so they would be used by the various testing efforts that are ongoing.

- The proximate cause of the feature David was seeing was the lookup table for the Sersic Hankel transform being a linear interpolation, so discontinuous second derivative.  Switching to a spline helped a lot.
- It improved even more when I switched to interpolating F(logk) * k^2, rather than just F(logk).  This was inspired by David pointing out that the asymptotic expansion of F is a/k^2 + b/k^3 +...
- I added code to estimate a,b as it is building up the FT values for the lookup table.  It now uses this approximation for the high-k limit (`k > _maxk`) once the approximation is found to be accurate at the `kvalue_accuracy` level.  This speeds up the code a lot, since it can stop building the lookup table much earlier than it used to.
- From #325, I took the additional photon shooting GSParams and the use of these in the Interpolants, OneDimensionalDeviate, etc. (This also accidentally pulled in some versions of Barney's `test_sersic_highn` scripts.  You can ignore them for this PR I suppose.)
- This also includes a switch to using GSParamsPtr, since the old way the GSParams were used in LRUCache actually had a bug.  It turned up during testing here, and I think it's the same bug that Barney reported in #422.  
- From #343, I took the improvement of adding explicit splits at the zeros of j0(kr) for the integrals.  This helps improve their accuracy and also speed up the convergence.
- Since I was doing all this work on the Sersic class, I also refactored the back end (the SersicInfo class) to simplify things a lot.  Mostly I switched it to use r0 as its basic unit rather than re.  I also broke up parts of what was the constructor into more discrete units, which meant that its use in SersicImpl was able to be simplified considerably.
- I added `table_spacing` as a new GSParams item.  It represents the spacing to use in lookup tables relative to the normal calculated value (by whatever calculation is done in each case).  So it has a default value of 1.  If it is <1, the lookup table will be more accurate at the expense of high time to calculate the values.
- I added `stepk_minimum_hlr` as another GSParams item.  For the stepk calculation, we typically start with a calculation based on `alias_threshold`.  But then, each class imposed some minimum value, e.g. 4*sigma for Gaussian.  These seemed rather arbitrary to me, so I regularized them with a multiple of the half-light-radius for all profiles that have this feature.  The default is 5, which roughly comports with the typical values that the various classes had been using.
- I added unit tests that `Sersic(n=0.5)` and `Sersic(n=1)` agree with `Gaussian` and `Exponential` respectively both in xValues and kValues.  (They didn't used to!)
- I changed the `test_rescale` unit test from using `wmult` to using `alias_threshold` and `maxk_threshold`.  `wmult` really only affects the effective maxk, not stepk.  So it is not generally sufficient to improve the flux values.  (i.e. The accuracy starts to level out above a certain value of `wmult`, since the relevant parameter that needs to drop becomes `alias_threshold`.)