Update data objects and some settings handling

[TomDonoghue]

This is an update to some data object handling and settings management (non-breaking changes).

Main updates:

• Data objects, that are all named tuples, are moved to a dedicated 'data.py' file
• Utilities for managing info & fields are moved to a dedicated fooof/core/info.py file
• Consolidate procedure for extracting & comparing attributes, generalizing into a 'get_info' and 'compare_info' function in fooof/utils
• Add 'FOOOFSettings' & 'FOOOFDataInfo' data objects, update the 'get_*' functions from the FOOOF objects to use the data objets, and add 'add_*' methods for updating FOOOF objects
• Split up and update data I/O procedures and data checking. Split up checking of data values and model regeneration, and general checker functions for loading from file or from data objects.
• Update combine_fooofs, to use new approach, and to fix issues it had when added objects did not have power spectra stored in them.
• Consolidate on consistency on settings ordering and use of tuples, etc

Overall, I think this is fairly straight-forward. Not totally sold on the name 'data.py' though, anyone have any other suggestions?

[voytek]

Left one comment, otherwise looks good!