Get the gains to acquire for the standard source from the lens paper data

[raacampbell]

Look in the lens paper data from the current directory and extract the PMT gains from each file. Then acquire at the same gains with the standard source. This will avoid us having to specify any gains at all at the CLI and it will make the hard-coded gain series file in the private directory for the record functions less critical.

[sumiya-kuroda]

I think mpqc.record.lens_paper right now saves file with only one PMT gain while the following analysis pipelines such as mpqc.analyse.get_quantalsize_from_file assumes there are files with different gains.

[raacampbell]

It does not assume multiple gains, I don't think, but it does look for the gain in the file name. I have not sorted this out yet. I think there is no strong need to do the lens paper at a range of gains, but it should be done at the same gain for all detectors.