[MNT]: Cleanup warnings

[asreimer]

It is common to see warnings like this when processing a file:

/opt/virtualenvs/dataproc/lib/python3.8/site-packages/tables/leaf.py:544: VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences (which is a list-or-tuple of lists-or-tuples-or ndarrays with different lengths or shapes) is deprecated. If you meant to do this, you must specify 'dtype=object' when creating the ndarray.
  key = numpy.array(key)
/opt/virtualenvs/dataproc/lib/python3.8/site-packages/resolvedvelocities/ResolveVectors.py:883: RuntimeWarning: invalid value encountered in true_divide
  mag_err = np.sqrt(ASA/AA)
/opt/virtualenvs/dataproc/lib/python3.8/site-packages/resolvedvelocities/ResolveVectors.py:907: RuntimeWarning: invalid value encountered in true_divide
  dir_err = np.sqrt(epep*ee*BSB)/(ee*epA**2-epep*eA**2)
/opt/virtualenvs/dataproc/lib/python3.8/site-packages/numpy/core/fromnumeric.py:3419: RuntimeWarning: Mean of empty slice.
  return _methods._mean(a, axis=axis, dtype=dtype,
/opt/virtualenvs/dataproc/lib/python3.8/site-packages/numpy/core/_methods.py:188: RuntimeWarning: invalid value encountered in double_scalars
  ret = ret.dtype.type(ret / rcount)
/opt/virtualenvs/dataproc/lib/python3.8/site-packages/resolvedvelocities/plot/summary_plots.py:195: RuntimeWarning: All-NaN slice encountered
  max_vmag_plotted = np.nanmax(vmag)
/opt/virtualenvs/dataproc/lib/python3.8/site-packages/matplotlib/quiver.py:668: RuntimeWarning: Mean of empty slice.
  amean = a[~self.Umask].mean()

Maybe we should clean these up?

Details needed to reproduce:

• Python 3.8 environment
• numpy 1.20.1
• resolvedvelocities v1.1.0-alpha
• File: IPY27/20140513.003/20140513.003_lp_10min-fitcal.h5