Use rough calibration if the calibrator's elevation was too far from the source's

[matteobachetti]

The elevation-calibration law calculated in SDTcal is a linear function, and it can be imprecise outside the fitted range, to the point that it could even get to negative values.
Now, the code will through a warning when this happens, and use a rougher constant calibration which should be more robust outside the validity range.

Also here:

• Exit cleanly if no valid calibration is present for a given channel (and suggest way to get a valid one)
• Better logging
• Fix for recent Numpy/Astropy/HDF5