Add "occupation_id" field to GravityObservations

[AdrianBenson]

Id for sets of readings taken during a single occupation

• Specified by user (i.e. read from excel) or
• Set automatically from breaks loop, station id's + threshold time gap

Why? Network adjustment requires repeat occupations, not repeat readings.

• We don't address this - except in network_map() method.
• Needed for qualitative assessment of solution quality.
• Station & loop summaries can include occupation count as well as observation count.

[craigmillernz]

Most of this already exists, but it would be useful to add the n_occupations to the loops tab of the excel output.