DB upgrade script corrupts data

[jamoore]

The OMERO3__5__OMERO3__6 updated script introduced in r1866 corrupts data with rounding errors while widening from float4 ("real") to float8 ("double precision"). This happens when the widening occurs so:

    alter table X alter column Y TYPE float 8;

Instead, the update has to happen via numeric thus:

    alter table X add column  tmp_Y double precision;
    update X set tmp_Y = (Y::numeric)::float8;
    alter table X drop column Y;
    alter table X rename tmp_Y to Y;

[jmoore]

r1893 corrects the script.

[jmoore]

Closing this ticket since the script is no longer dangerous (nor required). #803 has been pushed to milestone:3.0-Beta3 for more testing.