Failing nicely when cant fit the structure function

[twshimwell]

Presently when losoto tries to derive the structure function but cant find a good solution it doesnt fail in a very friendly way. This causes some issues for LINC (see ticket https://git.astron.nl/RD/LINC/-/issues/37) which produces the following error when trying to run the structure function sometimes.

usr/local/lib/python3.8/dist-packages/losoto/operations/structure.py:125: RuntimeWarning: divide by zero encountered in log10
par = np.linalg.lstsq( A.T, np.log10(variance[myselect][~mask]) )[0]
/usr/local/lib/python3.8/dist-packages/losoto/operations/structure.py:125: FutureWarning: rcond parameter will change to the default of machine precision times max(M, N) where M a
nd N are the input matrix dimensions.
To use the future default and silence this warning we advise to pass rcond=None, to keep using the old, explicitly pass rcond=-1.
par = np.linalg.lstsq( A.T, np.log10(variance[myselect][~mask]) )[0]
/usr/local/lib/python3.8/dist-packages/losoto/operations/structure.py:165: UserWarning: Attempted to set non-positive bottom ylim on a log-scaled axis.
Invalid limit will be ignored.
ax.set_ylim(ymin,ymax)
/usr/local/lib/python3.8/dist-packages/losoto/operations/structure.py:166: RuntimeWarning: divide by zero encountered in log10
ax1.set_ylim(np.log10(ymin),np.log10(ymax))
Traceback (most recent call last):
File "/usr/local/bin/losoto", line 154, in 
returncode += ops[ op ]._run_parser( soltab, parser, step )
File "/usr/local/lib/python3.8/dist-packages/losoto/operations/structure.py", line 16, in _run_parser
return run(soltab, doUnwrap, refAnt, plotName, ndiv)
File "/usr/local/lib/python3.8/dist-packages/losoto/operations/structure.py", line 166, in run
ax1.set_ylim(np.log10(ymin),np.log10(ymax))
File "/usr/local/lib/python3.8/dist-packages/matplotlib/axes/_base.py", line 4027, in set_ylim
bottom = self._validate_converted_limits(bottom, self.convert_yunits)
File "/usr/local/lib/python3.8/dist-packages/matplotlib/axes/_base.py", line 3614, in _validate_converted_limits
raise ValueError("Axis limits cannot be NaN or Inf")
ValueError: Axis limits cannot be NaN or Inf