support for exogenous CO2 prices from reference gdx

[Renato-Rodrigues]

Purpose of this PR

• add support for defining exogenous CO2 prices from the reference gdx.

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
• All automated model tests pass (FAIL 0 in the output of make test)
• The changelog CHANGELOG.md has been updated correctly

Further information (optional):

• Test runs are here:
  /p/projects/ecemf/tmp/test_exogenousPriceFromGDX/output/force_co2_2024-06-11_19.34.05

[ricardarosemann]

Thank you so much for implementing this :-)

[LaviniaBaumstark]

Why is this PR still open?

[Renato-Rodrigues]

It requires remind reporting changes that were still not implemented as described in the unchecked item in the check list.

[LaviniaBaumstark]

is this PR needed for the next release? Will it be merged very soon?