Executable python files and --jobs command line argument

- Made some of the python files executable
- Added --jobs -j to limit maximum number of parallel processes. I had some bad experience with a large project (my master thesis) where some parts required a huge amount of memory, after a clean; 8 pythontex processes would then grab 8 times this much memory. This option can also be useful when running on large clusters/servers, where you don't necessarily want to grab all 64 cores even though our document is large enough to do so.

Some options on the latex side should also be considered, a few cases:
- Mark the block as "do not run in parallel" (e.g. due to being memory intensive, or if utilizing some non-thread safe external library/application)
- Mark the block as "I will generate x parallel processes", then pythontex should handle this to avoid taxing the system.