io data integrity

[mwprestonjr]

Noticing several issues when saving and reloading ERPparamGroup objects:

• peak_mode check causes error when loading object without settings saved. See line 194 of fit.py
• uncropped_signals and uncropped_time do no survive saving and loading
• ERPparamGroup.plot errors for reloaded objects

io tests should be expanded to reduce these sorts of issues in the future