adjust readcoupled.R to support reading data from mif files

[orichters]

Purpose of this PR

readcoupledgdx.R could read from gdx, but comparing mif file data accross coupled runs is helpful as well, so if something does not look like a GAMS switch name, it loads from the mif, see tutorial.

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
• I checked that the in-code documentation is up-to-date
• All automated model tests pass, executed after my final commit (FAIL 0 in the output of make test)
• The changelog CHANGELOG.md has been updated correctly