Use integer literals where appropriate

[eumiro]

Python 2 needed explicit float literals in order to do the correct math. In Python 3 this is not needed anymore. Using integers describes better the intention of the code.

This PR only changes floats that end with .0 to integers where appropriate.

[LSchueler]

Hi eumiro,

thank you for your PR. Sometimes it makes code easier to read if float literals are explicitly used. Therefore, I don't think that converting every integer valued float to an integer literal is the right solution for GSTools and I'll close this PR.

[MuellerSeb]

I agree with @LSchueler