Fix tests for TimeStream

* tests/test_classes.py (test_read_metatdata_weird): We now raise an Attribute
error.
(test_timestream_init_bad_params): To trigger the exception we need to pass an
actuall string. "" is as if we didn't pass anything
(test_iter_by_timepoints_withgaps_no_rm_gaps): This no longer returns a None
image. Now we check to see if pixels is of length zero.
* timestream/__init__.py (TimeStream.__init__): We assing version to
self.version effectively running the necessary checks.