SNR was overestimated by a factor sqrt(2). Other minor issues were fixed.

Add error messages when the calibration database is empty.

Corrupted pairs with missing A or B SLIT resulted in unhandled
exception and premature exit. Now if is fixed, and it reports
in log that the pair is corrupted, but it go on with reduction
of following data.
In addition, previously the logger wrote to STDERR. Now it writes
to STDOUT.

If masterflat or masterlamp cannot be computed from the files observed
in the present night, they are copied from a previous night.
Before this fix str, nor and ext files derived from masterflat
and masterlamp were computed again overwriting previous files.
Now also str, nor and ext are copied from the database, if the
masterflat or masterlamp are copied.

It adds the new metadata.py with the function add_metadata.
Header keywords HIERARCH TNG DRS RED are added to reduced files.

DRS_MJD, HJD and BJD refers to mid-exposure time