test_alchemy fails for monetdb

[bartscheers]

Some tests do fail for MonetDB because in the current trap schema the SQL function median() is called, instead of the sys.median() function.

[bartscheers]

See discussio/approach in #482

[AntoniaR]

MonetDB has been retired from TraP.

This issue will likely be resolved in the new redesign of the source association and other SQL commands in TraP R7. Closing this issue.