[ENH] - Add triangle periodic fit mode

[TomDonoghue]

This adds a new "triangle" periodic fit mode.

Notes:

• The currently defined 'triangle' is a symmetric triangle or sawtooth shape
• It's defined to match the parameters of the gaussian (CF, PW, BW)
• However, there appear to be some quirks with using this in the existing fitting mode (e.g. bandwidth guesses don't seem great) such that it doesn't seem to end up on ideal solutions
• Currently, this mode should be considered experimental