Bring back ODMSample base class

[brimoor]

#117 went out a bit quick, but, upon closer inspection, I think we should not have deleted fiftyone.core.odm.ODMDatasetSample.

The current implementation has only fiftyone.core.odm.ODMSample and fiftyone.core.odm.NoDatasetSample, where the latter inherits directly from SerializableDocument:

fiftyone/fiftyone/core/odm/sample.py

Line 405 in 2730847

class NoDatasetSample(SerializableDocument):

I think there's still value in having the following hierarchy:

fiftyone.core.odm.ODMSample
    fiftyone.core.odm.ODMDatasetSample
    fiftyone.core.odm.ODMNoDatasetSample

where the base class fiftyone.core.odm.ODMSample defines the interface that all samples support. This will make it more clear what fiftyone.core.sample.Sample is allowed to do with its backing documents, for example.

Although NoDatasetSample is now completely home brewed (no MongoEngine), it is still a "document" in the sense that it is a JSON serializable representation of a Sample. So I see no issue with using the ODMNoDatasetSample name. It is in the odm package, after all.

[tylerganter]

I'm okay with this. The inheritance is ultimately messy no matter what we do, but I suppose this would be a bit cleaner.

[brimoor]

These definitions on NoDatasetSample are especially perplexing without having an ODMSample superclass that explains why they have to be there and return None

fiftyone/fiftyone/core/odm/sample.py

Lines 480 to 494 in 2730847

	@property
	def dataset_name(self):
	return None
	@property
	def id(self):
	return None
	@property
	def ingest_time(self):
	return None
	@property
	def in_db(self):
	return None

[brimoor]

This was implemented in #156