Define retention times for internal standards

[jorainer]

Define the retention times for the internal standards (listed in internal-standards.txt).

• Get the list of mzML files (newly measured samples from Vinicius?) and add a txt file with these.
• Write a .Rmd file documenting this analysis.
• Add the (approximate) retention time range to the internal-standards.txt file.

[jorainer]

Just as a template for a standart pre-processing that I do: https://github.com/EuracBiomedicalResearch/metabopd_disp_gesppark_plasma/blob/master/preprocessing_pos.Rmd

Would be nice if you could use a similar format (one hidden source block to define some global variables etc).