Pyyeti

- 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)

Minor update to the nas2cam tools to accommodate later releases of NX Nastran. Some wording and code cleaning as well.