Merge pull request #14 from allanderek/dev

Small tidy of pepa_ast.py
Fixes #11, now system equation does not have to have any operators

The fix is not very nice but for now it is all I can do fast.
Merge branch 'working' into dev
Cleaned up _name_subtree within pepa_treewalker, which also contained a * import which I have removed
A little more clean-up in pepa_treewalker.py to use the pepa_ast class 'asttype' members rather than hard-coding the string here, which is a little more fragile