Starting modularization of the library.

[loiseaujc]

Following what I discussed about in #61, this PR starts setting up the modularization of the package.
It includes the following changes:

• All the lowest-level routines, i.e.pass* and rad* subroutines, are now included in dedicated modules defining the appropriate interfaces.
• In preparation of the fypp-ification to support different kinds, the module fftpack_kinds has now been templated.

Some linting with fortitude has also been done.

Ping: @perazz, @zoziha

PS: Most of the changes are in fftpack_legacy_drivers_*. The rest essentially is just some formatting.

[codecov]

Codecov Report

❌ Patch coverage is 99.12358% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.24%. Comparing base (525e474) to head (70d9d9a).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fftpack_ifftshift.f90	0.00%	6 Missing ⚠️
src/fftpack_dct.f90	93.33%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #64       +/-   ##
===========================================
+ Coverage   81.16%   97.24%   +16.07%     
===========================================
  Files          68       50       -18     
  Lines        2140     2140               
  Branches      205      203        -2     
===========================================
+ Hits         1737     2081      +344     
+ Misses        403       59      -344     

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[zoziha]

Thanks, @loiseaujc . LGTM, and all tests have passed.

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[loiseaujc]

Hadn't realized that the search-replace rk to dp had been applied to the print. My bad.