More work on cmontecarlo.c and the unit tests

[mreineck]

cmontecarlo.c and cmontecarlo.h:

• some functions rearranged to allow better inlining
• specify "const" in the interfaces where appropriate
• add FIXME comments in places where I suspect problems but can't fix them myself
• minor performance tweaks (replacing divisions by multiplications etc.)
• unify variable declaration/definition where possible

Tests:

• avoid memory leaks in test_rpacket.c
• fix initialisations in test_montecarlo.c

[wkerzendorf]

@mreineck if you want to discuss again, bigger restructuring let me know. This looks very good - thank you!

[mreineck]

Hmm, the bound_free test still fails on Travis. Can't reproduce it locally. Xfailing it again...

[wkerzendorf]

travis is running on docker containers on ubuntu as far as I understand.

[mreineck]

I'm now done with the things that seemed most urgent to me. Performance-wise I don't see any more low-hanging fruit.
The cmontecarlo tests definitely need reworking though - but I fear I'm not really qualified for this.

[wkerzendorf]

looks good!