create minimum_version.cmake

[elidwa]

sliderule cmake and all plugins cmake files explicitly specify minimum version for cmake.
create minimum_version.cmake which would set it and have all cmake files include it, or do something similar. We should have only one place where minimum cmake is specified.

[elidwa]

I looked into it and it cannot be done without making sliderule cmake 'aware' of plugins cmakes which is not what we want. Cannot include common version cmake file since the minimum cmake version has to be defined before the first cmake command which would be 'include' command. Closing issue.