asynchronous option for cmult

Now has option to add en pin for asynchronous systems
cmult asynchronous option script change
feedback_osc addition, coeff_gen work

Added feedback_osc block to downconverter library. Allows generation
of complex exponential of specified frequency and with initial phase
offset using an initial vector that is rotated using complex
multiplication. A configurable number of calibration values are used
to control drift away from ideal over time. Will be used to generate
twiddle factors in fft direct and for DDCs. Allows BRAMs to be swapped
for multipliers.

cosin block updated to output fractions smaller than a 1/4 of a
cycle with some initial offset. Will be needed for fft_direct if
feedback_osc not required.

twiddle_general updated to use new coeff_gen interface. New
parameters need to be bubbled up and testing still required.