Implement Const-Correctness in Public API

[sbhutch]

As mentioned in the discussion tab, I have spent some time adding const qualifiers to a large portion of the public API to try and cut down on const_cast in C++ projects. I took the following approach:

• Break public API into modules
• For each module:
  • Add const qualifiers to initialization parameters in public API (coefficient arrays)
  • Add const qualifiers to input parameters in public API (input arrays)
  • Compile-and-fix loop to propagate const through the private APIs
  • Run autotests

Known issues:

• I could only compile for my system architecture, some SIMD implementations may have been missed
• I am not sure if cmake supports find-libfec yet, so may have missed the fun algorithms
• Breaking change: The user callback functions used in the framing module now need const qualifiers
• Not implemented for FFT run function as fft plan struct would need to be modified
• Not implemented for MRESAMP2 execute function as input array is used as ping pong buffer

Please let me know if you have any requests or concerns