Update to have more control on object clearing when adding data

[TomDonoghue]

Right now, if you use add_data to add data to an existing FOOOF object, it automatically clears the object of prior results & data. This doesn't anticipate the use case of re-loading a FOOOF object with just results, and then later adding data, to plot data and results together.

Suggested fix: add_data should have an optional argument clear, that indicates whether to clear prior results when adding new data.

Related ToDo: check through other add_* methods, add see if there are analogous issues, and related updates.

[TomDonoghue]

Something that is somewhat related (or the same kind of problem).

If an FOOOF object has data reloaded, without settings information, then settings end up as None. If one tries to explicitly add settings afterwards, with add_settings, this fails because an internal check fails on the currently defined null settings.

[TomDonoghue]

Should all be fixed now in #181