Rename PRTFILE argument to DATAFILE in fipreports

[daniel-sol]

In the fipreports submodule the positional argument is PRTFILE, while in all other submodules the positional argument is DATAFILE.
This is inconsistent, and creates some unnecessary code to deal with this fact inside of common and ecl2csv. This is a small change that will not make a lot of difference to the user, but will make the code neater.