Added ftau to standard b-tagging and c-tagging discriminants

[nikitapond]

Summary

This pull request introduces the following changes

• Allows for discriminant calculations to include ptau and ftau
• By default, ftau is 0, and ptau is only extracted from provided file if it exists
• Adds option to include ptau in mock file

closes #69

Conformity

• [x] Changelog entry
• [x] Documentation

[nikitapond]

For overall design choice when using the 'wps' script, currently when more than two fx values are required (e.g, xbb, or now single-b if taus are included), these are written to the output file as a single tuple. It might make sense to instead have this as a dict, so there is no ambiguity on which fx value is which.

[samvanstroud]

Awesome thanks!