[RF] remove cpdef in PmfGen

[skoudoro]

This PR is a small refactoring to remove the cpdef in pmf.pyx.

cpdef = def + cdef , so it does not make sense to have cpdef + cdef. it is either def +cdef or cpdef

I also use the opportunity to replace some pointer by memoryview. When using memoryview, we should make sure to use aligned memory by using [::1] for speed reason.

@gabknight, can you have a look and try?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (916757c) 82.37% compared to head (aa1768d) 82.37%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3063   +/-   ##
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  Coverage   82.37%   82.37%           
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  Files         146      146           
  Lines       20631    20631           
  Branches     3319     3319           
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  Hits        16995    16995           
  Misses       2805     2805           
  Partials      831      831           

[gabknight]

LGTM. This makes the code much nicer, thanks. Quick testing show no performance changes.

[skoudoro]

Thank you for the feedback! So I am going ahead and merge it