Change str.format() to f-string throughout the neuralprophet source code

[Kevin-Chen0]

For example:

log.info("{} NaN values in column {} were auto-imputed.".format(sum_na - remaining_na, column)) --> log.info(f"{sum_na - remaining_na} NaN values in column {column} were auto-imputed.")

This makes the code more readable as well as saves some space.

[Ben-Ostrovsky]

Hey Kevin, I would love to help with this issue! Which .py files in neuralprophet should be edited?

[Kevin-Chen0]

Yea that will be great Ben! Ideally, all of the files in the neuralprophet/ folder. Please look at the Contribute page here if it is your first time. Thx!

[Kevin-Chen0]

Close after successful merge of PR #773.