First parallel-block support
-Now supports basic uses of the parallel directive
-Fixed a case where background blocks were not initializing before the program exited
-Added a test case for the parallel block

Still not done:
-Make variable access and array aliasing threadsafe
-Fix an error case where the last statement in a parallel block is/encounters another parallel block
Merge branch 'master' of github.com:IanFinlayson/tetra