Add a custom LabMetaData object

[TomDonoghue]

It might be useful to add a custom LabMetaData object to the NWB files, upon which we can define additional fields and custom metadata to store in the NWB files.

Things that could be added:

Time alignment information, for example:

• approach: the strategy used to aligning timestamps
• separate_behav_comp: whether there was a separate computer running the behavioural task
• offset: the offset between timestamps as learned from the alignment model
• model_fit_score: the R^2 of the alignment model matching timestamps