Write new tests for conversions.py

[aaarendt]

The tests in test_conversion.py rely on the test database infrastructure that is no longer supported with our factory approach. All files and classes using the old approach are proposed to be deleted in #170. Note that these tests were already being skipped in the test suite.

This issue tracks conversations on how to rewrite tests of functions in conversions.py. Suggest many of these can be incorporated into test_point_measurements.py.