Rename poly_mult{,N}, generalize for arbitrary N,M.

poly_multN(M,N) can now multiply polynomials for arbitrary M and N.
Add alternative poly_mult1().
Add expression based functions in poly.lib.
Change tf?s_coeffs to return a single expression.
Simplify gpq a bit.
Rewrite lowpass3ec to use expressions more.

Rewrite lowpass3ec to use expressions more, specifically in when calculating the
filter coefficients.  Modify gpq in the same way, keep original as gpqN.  Also
introduce some helper functions in the process.
Rename lowpass3ec to bandsplit3ec.
Update test_poly and test_rmf.
Generate the sliders dynamically.
Add comments to .gitignore.
Ignore *.lv2 directories.
Remove doc compiling from build system.

Remove documentation compiling from the build system, since a) it already is not
generated by default, which is due to b) documentation generation via "faust
-mdoc" takes aeons anyway (it results in a 57 page PDF!).
Add four RM filter bank programs.

Add four RM filter bank programs:
- rmfbs_sym has *s*tatic/constant edge frequencies and uses a synthesis filter
  bank to reconstruct the signal
- rmfbs_sum has *s*tatic/constant edge frequencies and reconstruct the signal by
  summing up the bands
- rmfbd_sym has variable/*d*ynamic edge frequencies and uses a synthesis filter
  bank to reconstruct the signal
- rmfbd_sum has variable/*d*ynamic edge frequencies and reconstruct the signal
  by summing up the bands
These probably do not work well as VST, LV2, etc. plug-ins.  They probably work
best as JACK applications, or as extensions to programming languages, e.g. Pd.
Update and extend the README.md file.