Add possibility to read pars

[theHenks]

We need a pars reader which works in the same fashion as the metadata reader. It should respect the validity files and just read the propdicts out of the pars folder instead of the metadata folder.
It should work like

calib_pars = l200.par.cal(ValiditySelection)[detector]

The folder for cal currently contains then the period name and runs following. But, it would be nice to have the validity selection independent of the runs and periods. so the selection should be possible on the cal folder level. In addition, a feature like

calib_pars = l200.par.cal[period, run]

would also be nice to just get pars for a specific run.
I think this should be straightforward to implement.
Maybe I can make a draft PR and then we see how this looks like. Or @oschulz, do you have some time?

[oschulz]

I'll see that I do that next week - but remind me :-)

[theHenks]

Thanks to @fhagemann , there is already a draft version on the ceph. He implemented a similar functionality than we have for the metadata and config reader. Looks very very nice, so big thanks to Felix!

[oschulz]

Can we close this now?