Preprocess elseif constructs

[Schlevidon]

Summary

• IFDIFF can now properly handle the elseif keyword being used in a RHS by transforming elseif blocks into semantically equivalent if-else blocks.
• The elseif keyword appearing in a helper function is also handled.
• An elseif block can still be ignored just like any other if block.
• Appropriate tests for the new functionality were added.

Related Issues

• Closes Preprocess elseif constructs correctly #53

[pilarcoordinates]

Consider using figure(1), figure(2) etc. instead of repeated figure calls to avoid opening multiple windows.