Add merge_GSEA function

[DanielGarbozo]

I added the merge_GSEA function to obtain an output tsv file from all collections.
I change the documentation as well.

[danoguevara]

• Make sure the imported packages for your functions have the proper syntax within your function.
• You are missing adding the dependencies to the DESCRIPTION file. Do it by applying usethis::use_package("pkg1").
• Make sure to automatically generate the .Rd file of your function with the devtools::document() package.
• Avoid parsing arguments into the function (function.R <arg> <arg>) like a CLI tool. R functions are better used when applied directly like this: function(arg1, arg2).