[MNT] - Update Welch implementation

[TomDonoghue]

Responds to #357 (and related to #344). See #357 for notes and checks of the new implementation.

Updates:

• update the implementation of compute_spectrum_welch to use scipy.signal.welch
• remove our window padding options, as this is now available through scipy.signal.welch
• update the checker function, including updating the name (this is not public facing so I don't think should be an issue), and updating for the checker function to support the fast_len input
• removes the outliers_percent option from compute_spectrum_welch, as this is no longer supported using this implementation

[ryanhammonds]

looks good to me!