Added peptide data for the Dou et al. 2019 dataset

[cvanderaa]

@lgatto, when you have time, could you quickly have a look at the code for creating the Dou et al. 2019 data.

• inst/script/dou2019_* creates the data sets from author/paper supplied data (not exported on GitHub because too large)
• R/data/R contains the roxygen2 scripts for data documentation.

What do you think?

Here are the things I'm still considering:

• I export the data as peptide and protein data, maybe I should also add PSM data?
• Data is exported as separate objects, but we could use the power of Features to combine all in one object.
• Here, I combine different MS runs in one single MSnSet/SingleCellExperiment object (using MSnbase::combine(). Maybe it is better to keep different split apart (as we discussed previously)? Does Features also support a list of different MS experiments?

[lgatto]

@cvanderaa feel free to merge yourself