Use submodules

Description

Currently not fortran module is compiled using submodules. This makes the code unnecessarily harder to read and creates unnecessary rebuilds:

• Code is harder to read because there is little separation between interface and implementation
• Unnecessary rebuilds can occur as described in the commit message of 9c1e1be7
• Ideally there would be more separation of the interface and implementations, e.g. add an include folder where all the interface module are built, and all the submodules and "non-public" modules would be in src. Similar to C headers
• This would also help create an interface for octopus so that it can be called by other libraries and languages

Standards

• module interface files are named with a suffix _h e.g. module_h.F90
• submodule names are named as module_oct_m_impl for the main submodule, and freely named for subsequent ones
• in the interface module, add the module keyword before subroutine or function, and after pure, return_type, recursive, etc.

Proposal

• Moving forward, all new code will create separate module and submodule files with the previously mentioned standards:
• On new PRs, wen editing a module file that is not split, split the file before merging. Link to this PR for example on how to do so and what standards to follow

To discuss

• Naming convention of the submodules.
  • Some candidates: module_oct_m_impl, module_oct_sm, module_oct, module. All are valid as they are namespaces by module_oct_m.
  • Invalid: module_oct_m. Sometimes the compiler will complain about ambiguity
• Further separation into include and src subfolders
• Alternatives to using _inc.F90 in the module files
  • If we want other languages to use the interface module F90 files as headers, they should be as close to standard fortran as possible
  • Of course, the current #include should still work for fortran projects, but it is not guaranteed it will work for creating links to other languages

News snippet

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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.