Add Gaussian Fisher Transform Price Reversals indicator

[jealous]

The Gaussian Fisher Transform Price Reversals indicator, dubbed
FTR for short, is a stat based price reversal detection indicator
inspired by and based on the work of the electrical engineer
now private trader John F. Ehlers.

https://www.tradingview.com/script/ajZT2tZo-Gaussian-Fisher-Transform-Price-Reversals-FTR/

Implementation reference:

https://github.com/twopirllc/pandas-ta/blob/084dbe1c4b76082f383fa3029270ea9ac35e4dc7/pandas_ta/momentum/fisher.py#L9

Formular:

• Fisher Transform = 0.5 * ln((1 + X) / (1 - X))
• X is a series whose values are between -1 to 1

Examples:

• df['ftr'] returns the FTR with window 9
• df['ftr_20'] returns the FTR with window 20