Cleanup csds

The CSDS has grown a lot through the years and became more and more complex. In this merge request, I am simplifying greatly the API in order to cleanup the code.

Modifications:

• The module .c files are removed
• A single function is needed for the modules
• The logfile header is slightly modified and now includes which fields are used for which particles (along with the writing order).

Fixes #752 (closed).

src/chemistry/GEAR/chemistry_csds.c

-/*******************************************************************************
- * This file is part of SWIFT.
- * Coypright (c) 2020 Loic Hausammann (loic.hausammann@epfl.ch)
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Lesser General Public License as published
- * by the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU Lesser General Public License
- * along with this program.  If not, see <http://www.gnu.org/licenses/>.
- *
- ******************************************************************************/
-
-#include "../config.h"
-
-#ifdef WITH_CSDS
-
-#include "chemistry_csds.h"
-
-const char *chemistry_csds_field_names_part[chemistry_csds_field_part_count] = {
-    "ChemistryHydro",
-};
-const char *chemistry_csds_field_names_spart[chemistry_csds_field_spart_count] =
-    {
-        "MetalMassFractions",
-};
-
-#endif  // WITH_CSDS