Feature-reqiest: Backup old options.cfg before upgrade

[hajo4]

if installing a new version of pysol,
make a copy of the old file options.cfg to a filename like
options_old.cfg, options_v218.cfg or options_2024-09-30.cfg

[joeraz]

Not sure if this is needed:

1. Most version updates do not contain any significant updates to the options.cfg file.
2. Even when there are more significant updates, the changes are usually compatible with older versions. I tend to use both the last released version and current development version from time to time, and the options remain consistent. Worst case scenario, it's a single option that doesn't save right.
3. The options.cfg file contains the version number it was saved from, so if there ever is a more significant update, we can catch that and handle it appropriately.

[hajo4]

well, this request is because installing pysol 3.0 lost my settings from 2.18, see
#385

[joeraz]

I know.

I believe it's more likely there was an issue with the file before the upgrade than it is that the upgrade itself caused it. No changes are made to the options.cfg file by the upgrade.

[joeraz]

So I'm going to change this one around a bit. This logic has nothing to do with the upgrade. The config will be overwritten if there's a fatal error when trying to read it (i.e. the file is corrupt), or it's missing outright. If the config file is present and valid but still treated as invalid, that is a bug, and the focus should be on fixing said bug.

I'll look into what would need to be done to duplicate the old config as a log/reference file if PySol determines it to be invalid.