don't write rdata in test mode, improve start scripts

[orichters]

Purpose of this PR

• don't write RData in test mode: until now, ./start.R --test and ./start_bundle_coupled.R --test write .Rdata files in the REMIND main folder, potentially affecting running cascades. That is not what a test should do.
• facilitate selection of scenario config file: ./start.R NGFS_v4 will suffice
• print start.R warnings instead of just telling there were many
• improve error messages
• make sure no-one ever uses the c_whatever and cm_whatever simultaneously

Type of change

• Bug fix
• Minor change to start scripts

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

[dklein-pik]

Looks good. Thank you!