Fix bug in DCS filtering related to delays

This MR fixes issues with actuation stage calculations of transfer functions for FIR filtering. There were missing OMC to SUS delays not captured by the methods, and an incorrect dividing out when using the correct function. Likely the rationale for the latter was to preserve the phase for CI calculations.

This MR also include a change to the CI unit test DCS filter reference file: the old file was most likely created using the new pyDARM (not the O3 pyDARM version) and so it had an error already encoded into the reference DCS filter. I created a new file and saved only the actuation_tst filter to that file so the size of the file is much smaller than before.

I also made some small code cleanup adjustments that does not affect the functionality of the code.

Closes #271 (closed)