fix path_gdx_bau checks, some more realizations need it

[orichters]

Purpose of this PR

• we forgot in Check if input_bau.gdx is given or really needed; a couple of minor changes mostly related to MAgPIE coupling #1366 that three other realizations actually read from the input_bau.gdx.
• I first implemented it in a straightforward manner, but then the NAVIGATE config failed because this file is needed only if cm_emiscen = 1, so it all became a bit more complicated.
• without this change, the start scripts would set path_gdx_bau to NA and then the realizations lack this file and fail

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)

[dklein-pik]

Complicated but cool