Path processing in the Formula widget

**What's your use case?**
This would be most useful for _Spectroscopy_ and similar workflows.

Files always come with a path that carry information about organization. In `Formula` it is possible to do plenty of math but not much of text processing (except for the obvious string operations). However, it would be awesome to be able to extract the path and the filename using the `os` python module for example.

**What's your proposed solution?**

Make path handling functions available in `Formula`.

**Are there any alternative solutions?**

One can use string functions but that can become a bit awkward.

Start at the Formula widget and review how it currently exposes string operations and Python functionality. Clarify which path and filename operations should be available through the os module, then define completion as those functions being usable in Formula for extracting path information.

Written by the indexing model from the issue text.