Add proxybylevel option to chunkerflam and chunkerkerneval

[pbeckman]

Introduces a proxybylevel boolean option to chunkerflam and chunkerkerneval which, if true, adaptively computes the number of necessary proxy points separately at each level during relevant FLAM routines. This is needed for moderate to high frequency oscillatory problems (see #85), since the number of points needed to resolve oscillations in the kernel is smaller for smaller boxes.

I've tested this for Helmholtz (see the devtools/test/flamproxybylevelTest.m script), and it speeds up the FLAM rskelf and rskel routines on domains which are tens to a few hundred wavelengths by an approximately $O(k)$ factor.

This does require minor modifications to FLAM itself. I've forked FLAM under the fastalgorithms org and pushed my changes to https://github.com/fastalgorithms/FLAM. I'm not a submodule expert, so please let me know the best way to point the chunkie/FLAM submodule to this fork.