Standardise arguments

[William-N-Havard]

Expected behaviour
Praat.init uses file_path as keyword argument while Praat.to_file uses filepath. It would be better to use the same keyword argument (with underscore for both, or without for both) out of consistency.

It would be advisable to keep an optional legacy filepath or file_path argument (depending on which is chosen as standard) so as not to break existing code.