Force maven to compile with/for 1.6

Attempt to solve issues when compiling with
incorrectly set-up maven installations.
Add functionality for Permutations in rate Expressions

Permutations nPk=n!/(n-k)! are mathematical functions
that are requried as rate expressions in petri-nets
in some contexts.  Care is taken to avoid changing
existing code.
Merge branch 'stable-feature-perms' of github.com:maxf130/PIPE into stable-feature-perms

Fix messed up branching