Psf-utils

- changed ``--no-cache`` command line option to ``--refresh-cache``.

- Renamed *plot-psf* to *show-psf*.
- Improved support for DC operating points.
- Suppress access function names when printing members of PSF structures.
- Correct invalid units produced by Spectre on oppoint files (I, R, C).

Allow signal names to contain backslashes.

Improve support for DC operating points.

- Support PSF files that contain DC operating points.
- Support PSF files where values are given in a group.

- Allow, but ignore, properties on traces.