Correct `if` statement block and add comments

Correct `if` statement block in `optimise.order.recursive()`.

Add comments to reflect planned code changes.
Add code to calculate PageRank of system variables

Add code in `optimise.order.recursive()` to calculate PageRank of
variables in system of equations. Use PageRank to select the next
equation to add to the re-arranged system.
Add code to insert equations into `reordered` list

Add code in `optimise.order.make_graph()` to attach equations to
endogenous variables (including a warning if a variable has more than
one equation associated with it).

Add code in `optimise.order.recursive()` to add equations to the final
re-arranged list in `reordered`.

Update test case for `optimise.order.recursive()`.