Prepare to make input data available for externals. Dont stop on missing validation data.

[dklein-pik]

Purpose of this PR

Add a public repository to the list of input data repositories that are checked at startup. This public repo will contain the input data of the releases, except the validation.tgz, since we are not allowed to publish it. Thus, add the possibility to make individual input data files optional, in this case the validation.tgz. REMIND will not stop if these input data files are missing.

Type of change

• New feature

Checklist:

• My code follows the coding etiquette
• I performed a self-review of my own code
• I explained my changes within the PR, particularly in hard-to-understand areas
• The changelog CHANGELOG.md has been updated correctly

[LaviniaBaumstark]

did you run make test?
Will there be meaningfull info if downloading inptu data fails?

[dklein-pik]

did you run make test?

Yes. All tests pass in make test.

Will there be meaningfull info if downloading inptu data fails?

The following chunk, that is always part of the logfile, contains a message. A warning with the same message is printed at the end. Is that enough?

The log:

Load data..
  try https://rse.pik-potsdam.de/data/remind/public
    -> CESparametersAndGDX_d4c62a8f11e8a6827310519df09c98eb425a4070.tgz
  try /p/projects/rd3mod/inputdata/output
  try /p/projects/rd3mod/inputdata/output_1.27
    -> rev6.92_62eff8f7_remind.tgz
  try /p/projects/remind/inputdata/CESparametersAndGDX
Following optional files not found:
  rev6.92_62eff8f7_validationremind.tzz
..unpack files..
 -> rev6.92_62eff8f7_remind.tgz
 -> CESparametersAndGDX_d4c62a8f11e8a6827310519df09c98eb425a4070.tgz
..done

The warning:

Warning messages:
1: In download_unpack(input = files, targetdir = "input", repositories = repositories,  :
  Following optional files not found:
  rev6.92_62eff8f7_validationremind.tzz