Choose a better name for R the radial coordinate

[EmilyBourne]

Choose a better name for R the radial coordinate. In equations it is usually referred to as r and R represents something else. However in C++ capitalisation is usually used for classes while lowercase letters are used for variable names

[EmilyBourne]

After discussion with @gdgirard and @PaulineVidal we suggest Rminor and Rmajor in files where there may be confusion (e.g. simulations). In files which are specific to a known geometry R is permitted. E.g. R is permitted in a class handling a polar plane only