Fix nuclear units in declaration strings

[aodenweller]

Purpose of this PR

This PR fixes several nuclear-related units in the declaration strings of parameters and variables. It doesn't change any code.

In core/declarations.gms

• The unit of pm_eta_conv is TWa_elec/Mt_Ur for nuclear
• The unit of pm_PEPrice is tr$2017/Mt_Ur for uranium

Similarly, in 31_fossil/declarations.gms some units were wrong. Please check these in particular, maybe @LaviniaBaumstark?

I also fixed the reporting in remind2 accordingly: pik-piam/remind2#806

Type of change

Parts concerned

• ◻️ GAMS Code
• ◻️ R-scripts
• ☑️ Documentation (GAMS incode documentation, comments, tutorials)
• ◻️ Input data / CES parameters
• ◻️ Tests, CI/CD (continuous integration/deployment)
• ◻️ Configuration (switches in main.gms, default.cfg, and scenario_config*.csv files)
• ◻️ Other (please give a description)

Impact

• ☑️ Bug fix
• ◻️ Refactoring
• ◻️ New feature
• ◻️ Change of parameter values or input data (including CES parameters)
• ◻️ Minor change (default scenarios show only small differences)
• ◻️ Fundamental change of results of default scenarios

Checklist

• I executed the automated model tests (make test) after my final commit and all tests pass (FAIL 0)
• I adjusted the reporting in remind2 if and where it was needed
• I adjusted the madrat packages (mrremind and other packages involved) for input data generation if and where it was needed
• My code follows the coding etiquette
• I explained my changes within the PR, particularly in hard-to-understand areas
• I checked that the in-code documentation is up-to-date
• I adjusted forbiddenColumnNames in readCheckScenarioConfig.R in case the PR leads to deprecated switches
• I updated the CHANGELOG.md correctly (added, changed, fixed, removed, input data/calibration)