Generate the correct index for a non-mixed arg

When generating kernel argument names for mixed ParLoops we were
assuming that all Dat arguments were mixed, though that is not
necessarily the case. Fix that by using index 0 for non-mixed ParLoop
arguments.
Correctly deal with an empty params list in pytest
Add tests for incrementing into MixedDat from Dat
Merge pull request #281 from OP2/fix-code-gen-for-non-mixed-args

Generate the correct index for a non-mixed arg

When generating kernel argument names for mixed ParLoops we were
assuming that all Dat arguments were mixed, though that is not
necessarily the case. Fix that by using index 0 for non-mixed ParLoop
arguments.