Rename functions starting from under score

[yasahi-hpc]

Improves #81

Following functions have been renamed

_get_shift => get_shift
_roll => roll_impl
_fftshift => fftshift_impl
_ifftshift => ifftshift_impl
_normalize => normalize_impl
_coefficients => coefficients_impl
_crop_or_pad => crop_or_pad_impl
_prep_transpose_view => prep_transpose_view
_transpose => transpose_impl
_create => create_plan
_init_threads => init_threads
_exec => exec_plan
_destroy_plan, _destroy_info => destroy_plan_and_info

After review, following changes are made.

_roll => roll
_coefficients => get_coefficients
_destroy_plan => destroy_plan
_destroy_info => destroy_info

[cedricchevalier19]

I would suggest not to use _impl names in the Impl namespace.

[yasahi-hpc]

I would suggest not to use _impl names in the Impl namespace.

Thank you for your review.
I have renamed _roll and _coefficients functions.

Some functions like normalize_impl are unchanged because normalize is the name of (internal) API and it conflicts.
Would it be better to rename normalize_impl to normalize_exec, for example?

[cedricchevalier19]

I prefer _impl than _exec.

[yasahi-hpc]

I prefer _impl than _exec.

OK. I will merge this. Thank you for your review.