Filenames: adjust prefix/suffix

- Since OpenDSS v8 (initial PM release), many filenames are now appended with a suffix to disambiguate the outputs from each actor
- For some outputs, the casename is also used in the filenames or paths.
- From the general posts and information publicly available on the internet, usage of the internal PM mechanism seems limited

I propose we set the default suffix to empty when PM is not active, and leave a compat flag to keep the old behavior.

We can also expose some functions to handle the DSSContext names or custom suffix etc.