detselect logic is wrong in some cases

[eugene]

detselect generates SQL that does not really make sense for the following cases:

• no time constraint is provided : in this case, it is clear that all detRuns should potentially match. in fact, the sql generates where conditions of the form (time_begin IS NULL OR time_begin IS NULL). it should just drop the time_begin and time_end conditions

• exptime supplied, but detRun exp_time_max or min are NULL : if one of these values is null, that should represent no constraint. however, detselect generates sql forcing a range check (exp_time_min < ...). this sql should include conditions of the form 'OR exp_time_min IS NULL'

[eugene]

fixed the exptime constraint (and all other float filters) : the search looks for, eg, (exp_time_min < value OR exp_time_min is null)