class_maker.py should better support new versions of existing algorithms

[mantid-builder]

This issue was originally TRAC 9674

Original Reporter: Keith Brown

I've just tired to create a new class file for http://trac.mantidproject.org/mantid/ticket/9501 using the class_maker.py file, however it didn't run properly as there was already an rst file for version 1 of that algorithm. In the end I had to add the --no-rst flag so i could create the class.

update the script to better support the creation of new versions of existing algorithms

[NickDraper]

The frequency of this is not high enough to put the effort in.

wontfix