Updated ints to floats in README example.

See merge request !5

Switched storage order in "States" representation of an epidemic.  We now have TxPxS for Time, Population (size), States.

# Conflicts:
#	pyods/__init__.py
#	pyods/model/Model.py
#	pyods/model/Rate.py
#	pyods/model/State.py
#	pyods/model/Transition.py
#	pyods/simulator/Simulator.py
#	pyods/simulator/__init__.py
#	tests/test_basic.py

Re-write of the DAG generation algorithm after rewrite of semantic analyser.  Need to implement for STMs, I think.

Small tweaks to ASTNode, adding fields to record a node's current scope, and evalType to record what type an expression returns.

Deleted now redundant inject_data.py file.

Extensive review of the semantic analysis steps to try to include type checking.  Semantic analysis now has its own folder with gemlang, scopes are now encapsulated within a SymbolTable object, TreeWalker updated to climb up a type hierarchy where AST Nodes inherit from parent nodes.  This allows us to match generic types of node.

Simplified symbol table slightly, and fixed bug in dag generator caused by IdRef definition pointing to IdRef nodes rather than *Assign nodes for formal arguments.

Rewrite of the FunctionSymbol system, differentiating formal arguments from config arguments in STMSymbol.

Still not a great interface to symbol declaration and resolution phases.  Answers on a postcard...

Fixed bug in ASTWalker -- the tree was being modified in-place, rather than separately.  Now the ASTWalker works on a copy of the tree to avoid mangling existing tree objects.  Shallow copies of nodes are made to avoid copying any external references (e.g. symbol table).

Fixed doc and refactored ModelGenerator -> CodeGenerator to more accurately reflect what the class is doing.