PSDFirKernel: clean up

- remove redundant operations from linear_phase_fir_kernel_to_minimum_phase_whitening_fir_kernel().  lots of rolling arrays one way, then back again, multiplying by normalization constants and taking them back out again.
- NOTE:  there was probably a bug in this code, roll-left and roll-right operations were not the inverses of each other, leading to a net shift of the frequency series samples.  re-testing of the zero-phase whitening filter will be required after this change.
- NOTE:  this also fixes the returned phase vector's length