Change optional dataset properties to default arguments

[kmdouglass]

The DatabaseAtom code should be cleaned up by requiring only acqID, data, prefix, and datasetType as positional arguments; the rest can be made arguments that default to None.

This should help ease code maintenance and the addition of more fields in the future.

I'm working this at the same time as #23 since the creation of a SimpleParser should reflect these changes. I'm also adding a date property as an optional field.