Add methods to deal with missing required properties: d7eba0d

Here are a few edge cases I could not quite figure out how to deal with:

For TimeSeries:
timestamps and starting_time are optional datasets. However the rate attribute of starting_time is required, but only if timestamps are missing?

For ImageSeries:
data is required, but not when external_files are specified, ref this:
NeurodataWithoutBorders/nwb-schema#462

Failing tests after enforcing required properties on export

• tests.unit.anonTest/testAnonDataset
• tests.unit.linkTest/testExternalResolution
• tests.unit.multipleConstrainedTest/testRoundabout

For TimeSeries:
timestamps and starting_time are optional datasets. However the rate attribute of starting_time is required, but only if timestamps are missing?

The rule is: TimeStamps must have (timestamps) xor (starting_time & rate). If starting_time exists then rate must also exist, which is why it is a required property of starting_time. The schema language does not have a way of expressing an xor condition, so we don't have a way to formally indicate that you must have EITHER timestamps OR starting_time. That constraint must be specified manually by the API.

For ImageSeries:
data is required, but not when external_files are specified, ref this:
NeurodataWithoutBorders/nwb-schema#462

Yes, this is a tricky edge-case. Since ImageSeries isa child of TimeSeries, the data field is required (children cannot un-require a field). However, when you are using external mode it does not make sense to add data. This would ideally be fixed by reorganizing the parent classes in a way where you wouldn't require data when you don't want to, but the current work-around is to create an empty or near-empty dataset.