Print out messages explaining star selection

[julienguy]

This PR in response to issue #1066.
The flux calibration selection of stars is now more verbose.
Example for frame-r6-00068675.fits

INFO:fluxcalibration.py:95:main: compute flux calibration
INFO:fluxcalibration.py:111:main: apply cut |delta color|<0.1
INFO:fluxcalibration.py:117:main:  discard 1 stars with dcolor= [0.19936935785406895]
ERROR:fluxcalibration.py:142:main: selection cuts discarded all stars

The delta color test is based on the color of the best fit stellar model. Here a delta color of 0.2 indicates a pretty bad fit.
The color difference can be larger than the initial color selection range because the standard star fitting code includes more stellar parameter grid nodes than given by the script color selection cut in order to have a well behaved input grid geometry (in parameter space).