Add datafiles_root configuration value

[gjover]

One can define datafiles_root in pytest configuration file to define the datafiles root directory.

This feature allows to define a common root folder instead of defining the FIXTURE_DIR on each test.

[omarkohl]

Thank you for your contribution, I will have a look shortly!

[omarkohl]

I only had a brief look, but if I'm understanding correctly you are adding a new option. Can you provide an example where that would be useful? Would you be willing to also update the documentation?

[omarkohl]

No reply from OP.