Add support for managing frequency bands

[TomDonoghue]

It is still useful / required to sort peaks into frequency bands. To manage this in other projects, I've used a simple 'Bands' object, which manages frequency band definitions, and I think that should be added into FOOOF directly.

To Do:

• Add the Bands object (as has been used in the MEG projects
• Add support for using the band object, for example in analysis, and in the FOOOF averaging in Averaging across FOOOF results #135