Add PWRXYZ: Prefer use <module> over include <source file>

[alvrogd]

Just the entry and example codes. The benchmark will be added soon.

[RRiva]

Hi @alvrogd, in general this is a nice idea, but sometimes include is exactly what is needed. A use case of include is to implement poor man generics. For example:

functionmat_op(a) result(y)
real:: a(..)
real:: y
select rank(a)
rank(0)
include'long_expression_that_is_rank_agnostic.f90'
rank(1)
include'long_expression_that_is_rank_agnostic.f90'
rank(2)
include'long_expression_that_is_rank_agnostic.f90'
end select
endfunction

A similar, more concrete, example is provided at this implementation of a generic list. In this case, the code depends on a compiler directive. Therefore, it cannot be compiled and used until that is specified.

Just to be clear, there are plenty of cases where this PR makes sense (mostly old F77 code), but distinguishing the cases where include must be kept might be difficult.