Issue789medianValuesFailingInDevVersion

Replaced median aggregation function with simpler code (from https://gist.github.com/rdmurphy/3f73c7b1826cacee34f6c2a855b12e2e). This is still PostgreSQL-specific, but no longer requires the special median migration to work. References issue #789

Updated changes file [skip ci]. References issue #789

Moved ABC import so that Ed's comment about sphinx autodocumentation still makes sense. References issue #789