Mark `pure` subroutines and functions as `pure` where possible

[sbryngelson]

https://stackoverflow.com/questions/30264729/proper-use-of-the-pure-keyword-fortran

https://stackoverflow.com/questions/9813494/pure-subroutines-in-fortran-compiler-optimization

It is a good idea to mark pure subroutines (without side effects) as pure. This means that the routine only has intent() and does not modify module variables (which is a side effect).