Minor error fixes

* In ODM2LabAnalyses, changed the the cardinality of the foreign key
from FeatureActions to SpecimenBatchPositions to not allow 1-to-many.
SpecimenBatchPositions is a subclass of FeatureActions, and the two
tables share the same primary key.
* Changed Results.IntendedObservationSpacing to be nullable as it is
only applicable to Results that are TimeSeries Coverages.