ENHANCEMENT: Avoid if_else_isinstance(Subsample) in regression.py code

[tmorzade]

Is your feature request related to a problem? Please describe.
There are if_else switches according to "cv" value in the regression.py code that make the code unclear and could become more and more cumbersome with new methods.

Describe the solution you'd like
Use the matrix k_ for all methods

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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