bug in calculation of time axis for MS ObsCore metadata

[d3v-null]

in ObsCoreMeta.from_visibility, t_xel ("Number of elements along the time axis.") is given by:

Karabo-Pipeline/karabo/data/obscore.py

Line 545 in 420469a

ocm.t_xel = MSMainTable.nrows(ms_path=vis_inode)

According to the test, this should be 24

Karabo-Pipeline/karabo/test/test_obscore.py

Line 123 in 420469a

number_of_time_steps=24,

but if we update the test to check for 24 time steps, the test fails because t_xel is 15120

expected_timesteps = 24
if visibility.format == "OSKAR_VIS":
  observation = Observation(..., number_of_time_steps=expected_timesteps)
...
assert ocm.t_xel is not None and ocm.t_xel == expected_timesteps

What we really want is the number of unique values in the time array. this passes:

ocm.t_xel = len(np.unique(MSMainTable.get_col(ms_path=vis_inode, col="TIME", nrow=300000000)))

I intend to fix this in an upcoming PR that also ads uvfits support.