don't let cs2 fail if outputdirs are not unique

[orichters]

Purpose of this PR

If you accidentally pass an output directory multiple times to cs2, then it fails to find the correct files and fails, because lucode2::getScenNames() returns the scenario name only once, leading with

outputDirs <- ../remind-2023-06-12/output/C_h_cpol-rem-6, ../remind-2023-06-12/output/C_h_cpol-rem-6, 
output/C_h_cpol_d50-rem-5, output/C_h_cpol_d95high-rem-15

to this error:

Error in `normalizePath()`:
! path[2]="../remind-2023-06-12/output/C_h_cpol-rem-6/REMIND_generic_C_h_cpol_d50-rem-5.mif": No such file or directory

After normalizing the path, unique is applied, avoiding the problem.

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)
• No need for adjusting the changelog CHANGELOG.md