move specific functions from V3 chx_generic_fn to specific modules

[ambarb]

If we do this, they become easier to find for a common library and are seen to by a wider audience. We should keep the docstring attributes to Yugang Zhang (@yugangzhang).

Stopped for now because this is starting to seem like a V3 problem.

Table of functions and where they can go

Function	Move to	Comments
sgolay2d	_commonspeckle. or skbeam.core.utils
apply_mask	_commonspeckle or skbeam.core.roi
get_max_countc	_commonspeckle or skbeam.core.roi	roi has some of these functions, but for frames. same function is in chx_speckle*

high level plotting functions that may be generally useful to set aside

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