Change intent(inout) to intent(in) in many places

Description

Replaced many intent(inout) statements with intent(in), as the corresponding arguments were not modified. Also removed the use of global variables in the forces_oct_m module, as this was not really necessary and required some arguments to have the target attribute, which in turn could prevent the compiler from enforcing an intent(in).

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.