Use technical term 'centrally symmetric' for 'point symmetric'

[schillic]

According to Wikipedia, what I called point symmetry is weaker than central symmetry:

An object that is invariant under a point reflection is said to possess point symmetry; if it is invariant under point reflection through its center, it is said to possess central symmetry or to be centrally symmetric.

@mforets:
Should we clarify by merging this PR?
Should we also rename the interfaces?

[mforets]

Should we clarify by merging this PR?

ok

Should we also rename the interfaces?

👍 , those interfaces have a center(...) method, so i think it is good to use Centrally in their names.

[schillic]

I renamed the interfaces, good to merge from my side!