Handle array_get better for multi-dim arrays

[sjoelund]

The code generator can now generate constant indexes for ASUB expressions. The 2D, 3D and 4D-specific array accessors have now been removed (except for String arrays which did not implement these for some reason).

[OpenModelica-Hudson]

The job failed the coding style; for details, see OpenModelica_TEST_PULL_REQUEST 2018-02-13_08-36-56. Remember to clean up the commit you did that breaks the coding style rather than piling a new commit on top of the old ones. This goes double for accidental addition of files that you need to remove.

[OpenModelica-Hudson]

The test suite is unstable according to OpenModelica_TEST_PULL_REQUEST 2018-02-13_08-43-12.