Merge FIR filter generation code into pyDARM

It has been proposed that we try to move as much of the calibration codebase into pyDARM as is possible. I believe we can merge the FIR filter generation code into pyDARM since it relies primarily upon the transfer function computations calculated therein and tuning of FIR filter parameter coefficients (though I don't know what external packages are needed for that). Perhaps there is some fundamental reason this cannot be done.

Thoughts?