FFT compressor improvements

[cjrolo]

Currently the FFT compressor has a different signature than all the other compressors. One idea was to get the optimizer to do some work and set the FFT parametrization via other means.

So, the rational for this PR is the following:

1. FFT have the same base function signature that the others, so a call to FFT will have a sane default
2. New FFT signatures for dealing with either set frequencies or maximum error allowed
   • This can be called explicitly when required without interfering with a normal flow of the code (They are exceptional cases anyway)

Bugfixes:

• FFT ID variable name
• Greatly increased the performance of the FFT calculations for best error approximations (Still heavy)

Tests were adjusted for the new signatures and new defaults.

[rukai]

All very minor issues except the calculate_error issue which will tank your performance unless the compiler pulls off some heroic optimizations.

Feel free to merge now and then investigate these in a followup PR.