dmdt parameters can be misleading without errors in faint objects

[mnicholl]

A faint/noisy event can show random scatter of >~0.1 mag; it's currently difficult to differentiate this from a real rise using dmdt in objects table. E.g. an event like this might appear to be rising in dmdt: https://lasair-iris.roe.ac.uk/object/ZTF20acnzozv/ , but really is consistent with random scatter. Sometimes with short time intervals the implied dmdt can be very steep!

I think this can be solved using a parameter such as err_dmdt = sqrt(sigmapsf_1 ^2 + sigmapsf_2 ^2) / delta_t, allowing query to include err_dmdt < (user preference)