Code modernisation

[aosprey]

There are still some outdated language features in the code that should probably be updated for i) future-proofing and ii) ease of use.

• #ifdef : Some of this functionality can done with IF clauses in the code, and different versions of routines if required, but this may mean specifing paramters at runtime versus compile time. The #ifdefs generally occur at a higher level (now that the CAM coupling has been removed) so it probably wouldn't affect performance.
• DATA : The way these are used, we can just replace with normal array assignment.
• RETURN at end of SUBROUTINE blocks : This is redundant.
• CONTINUE and GOTO in loops : We can re-write these with a standard loop structure.

[aosprey]

Partially solved by above branch. Goto and continues are more complicated - so moved to #28