Specify retention times of standards observed in serum samples

[jorainer]

The goal is to update the retention times of the standards in the standards_dilution.txt file by looking through the EICs generated in issue #22.

Add also additional columns to the table:

• "data_set": the name of the data set on which the rt was defined, e.g. "2016 water" or "2020-standards-serum".
• "sample": information on the sample in which the rt was measured (or in which the standard was dissolved). For example "water" or "QC CHRIS".
• additional columns with TRUE/FALSE for specific peak/data characteristics seen in the EIC. For example "single_peak" to characterize compounds for which a single peak or multiple peaks were observed at the reported retention time, or "stable_rt" if the retention time for low and high concentration was the same. Ideally, the column should be named so that TRUE would mean good quality, FALSE not so good. That way it would be easier to calculate a final score for the rt.
• "operator": the name of the person who defined the rt.

[mar-garcia]

Partly done in pull request #25 It is still missing: add a column with the peak shape quality.

[mar-garcia]

Updated columns with peak shape quality