Pyyeti

This release is primarily for fixing some minor bugs/issues and updating the documentation:

- fdepsd has been updated to fix an issue that can cause an erroneous "ski slope" to appear in the lower frequencies. Many thanks to Vince G. for pointing this out! :-)
- as part of the fix, added the "detrend" option
- also made it mandatory that the optional arguments must be named

- Updated the CSTM output from the op2 reader to ensure it is a floating point array and not an integer array. Many thanks to Jeremy P. pointing this out! :-)

- Fixed an issue in cyclecount.getbins when there is only one amplitude (or mean) value after rainflow counting. This would cause "sigcount" to fail, for example, on this: ``cyclecount.sigcount(np.array([10, -10, 10, -10]))``

- demean signals by default
- use "unbiased" correlations
- improved usage of scipy.signal.csd function so it *can* be equivalent to time domain approach
- added example of using the output directly

Made pyYeti more pathlib.Path aware.

Made random number generator usage (in psd.psd2time and ytools.eig_si) definable by analyst. This is good especially for parallel processing tasks.

Other minor documentation updates

Made some significant updates to the ERA class (eigensystem realization algorithm).

- Added the output EMAC indicator (EMACO) which may be superior to the normal EMAC
- Added the related CMIO indicator
- Fix bugs
- Added more features (configurable Hankel matrix size, configurable figure labels, the verbose option)
- Simplified some coding

Also added a beta version of the Natural Excitation Technique (NExT) for preparing ambient vibration data for ERA. Implemented both the time domain and frequency domain with windowing approaches to compute cross-correlations.

- Update `cb.cbcheck` to handle null rows/cols more robustly:
- chops out null rows/cols if present in both K & M
- statically reduces K for massless DOF

- Vectorize `nastran.n2p.getcoordinates`. Thanks rjyunis! :-)

- Minor updates to `ytools.eig_si`

- Doc updates

Minor release:

- to update the documentation format
- to allow for an empty 13th field on a Nastran CORD2* card
- updated the op4 ascii format reader & writer to be able to handle larger matrices (added in v1.2.4)