Corrections on concatnc

[abierjon]

In concatnc.F90 :

• one should increase the length of the strings for the variables "file", "tmpvar", "tmpfile", "title", "units" declared at the beginning of the main program (to keep the entirety of the strings stored in the input files, especially the titles)

• dealing with stats.nc files as inputs, the time coordinates must be put from Local Times to sols before being used (in the main program and in the subroutine change_time_axis)

• prevent the conversion to a Ls time axis when dealing with stats.nc files (since they don't store the variable "controle" that gives the starting sol of the run)

[abierjon]

POULDP 09/07/2020 : concatenating stats files doesn't make much sense, so it would be preferable to properly end the program when a stats file is detected => replace 2nd and 3rd items

The 1st item on the strings' length is still to be done too.

[abierjon]

One should modify the GCM doc accordingly to these changes too (especially concerning the stats files that are not handled by the program)

[abierjon]

1st item on strings' length is done for concatnc.F90, localtime.F90, solzenangle.F90 by r2434

Revision r2434 also enables to properly concatenate stats files, since it has finally been decided to keep this possibility.

3rd item on preventing Ls time axis conversion when dealing with stats is still to be done.