Fix incorrect variable name in documentation

Change-Id: I312e3886ebc692f2331ac2f9a612d530b5d4914c
Fix DD load balancing bug with GPU sharing

The recent DD load balancing fix which solved the issue of incorrect
imbalance measure with GPU sharing (ba8232e9) addressed GPUs with
incorrect indexing. This caused out of bounds indexing in the GPU ID
query function. The query function also had a bug in the error checking
which allowed the incorrect indexing.
Now also mdrun -nb cpu -gpu_id ... is allowed, which before would give
a fatal error.

This commit addresses both issues; fixes #1385

Change-Id: I2800f610b873da92afe78bbfd869258f378ba2d7
Merge branch 'release-4-6' into comp_el_r46