make sure NDC run has not lower taxes than BAU

[orichters]

Purpose of this PR

• close https://github.com/remindmodel/development_issues/issues/236
• NDC always has a BAU run in order to calculate an emission target for those countries that have no NDC at all. It makes no sense if NDC has lower carbon prices than BAU
• Load bau prices and use them as a minimum value
• also load historical data in case someone uses NDC module for <= 2020

Type of change

• Bug fix

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
• I checked that the in-code documentation is up-to-date
• All automated model tests pass (FAIL 0 in the output of make test)

Further information (optional):

• Test runs are here: /p/tmp/oliverr/remind/output/SSP2EU-NDC_2024-02-22_13.35.12: just one iteration to see whether everything works