not writing enough times should issue a warning at exit

[doutriaux1]

if user define a time axis of length 10 but writes only 2 time steps, cmor should issue a warning when the file is closed (you decalred 10 times but only wrote 2)

[mauzey1]

@doutriaux1 I made a check to find out when the ntimes_written of a variable is less than the length of the time axis. However, if cmor_write wrote data with ntimes_passed=NULL, the ntimes_written value will be zero even though there is a non-zero length for the time axis.

According to the documentation for cmor_write:

[ntimes_passed] = integer number of time slices passed on this call. If omitted, the number will be assumed to be the size of the time dimension of the data (if there is a time dimension).

Should we set ntimes_written to the length of the time axis in this case?