Adjust models.MESSAGE for iiasa/ixmp#212

[khaeru]

iiasa/ixmp#212 adds a feature to the ixmp.model.GAMSModel class, allowing a temporary directory for input and output GDX files, listing files, etc.

This feature is used for e.g. ixmp's DantzigModel; but it is not desired for MESSAGE models, where the user may want to inspect the GDX files. This PR explicitly disables the feature for MESSAGE, MESSAGE_MACRO, etc.

How to review

• Confirm CI passes.
• Confirm MESSAGE models can be run with this branch and the branch in Add Model.initialize ixmp#212 (fyi @behnam-zakeri)

PR checklist

• Tests added. N/A, no change in behaviour
• Documentation added. N/A
• Release notes updated. N/A

[codecov]

Codecov Report

Merging #311 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #311   +/-   ##
======================================
  Coverage    79.8%   79.8%           
======================================
  Files          13      13           
  Lines        1144    1144           
======================================
  Hits          913     913           
  Misses        231     231

Impacted Files	Coverage Δ
message_ix/models.py	98.14% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 45e630c...5efb731. Read the comment docs.