NonRecognizedPassband.__init__(): add 'path' and 'keyword' arguments
Include FITS image and keyword in NonRecognizedPassband.__str__()
Improve docstring of NonRecognizedPassband.__init__()
Improve docstring of NonRecognizedPassband.__str__()
FITSImage.pfilter(): pass path and keyword to NonRecognizedPassband
Fix the NonRecognizedPassband error message raised by FITSImage

The error message of NonRecognizedPassband was improved in commit
85751393fc4d and its __init__() method now receives only the name of
the photometric filter. However, the NonRecognizedPassband exception
raised by the FITSImage class was not updated — an error message is
passed to NonRecognizedPassband, which interprets it as the name of
the filter and results in confusing messages such as:

  passband.NonRecognizedPassband: cannot identify the photometric
  system of filter 'ferM_0013.fits: unknown filter 'error' ('INSFLNAM'
  keyword)'. If this is a legitimate filter name, and you think LEMON
  should be able to recognize it, please let us know at
  http://github.com/vterron/lemon/issues

Fix this by adding two new keyword arguments, 'path' and 'keyword',
to NonRecognizedPassband.__init__(), so that the error messages can
include when applicable the path to the FITS file and keyword from
which the unrecognized photometric filter was read.

This merges branch 'fix_NonRecognizedPassband_msg'
Add ASTROMETRY_COMMAND module-level variable
Add exception AstrometryNetNotInstalled
Add exception AstrometryNetError
Add astrometry_net() module-level function
Add docstring to astrometry_net()
Use astrometry_net() instead of astrometry()
Remove module-level function astrometry()
Remove no-longer-necessary options
Update module output
Update comments write to the FITS header
Remove unused imports
Update module docstring
Remove module-level function _ahead_file()
Remove module-level function scamp()