Add kreg bw option

[john-p-ryan]

By utilizing the bw argument in the KernelReg function, we can avoid the "spaghetti" problem in the weights by smoothing the mtr estimates. As described by @jdebacker, a bw of 120,000, we can fit the data but avoid the oscillations.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.37%. Comparing base (10985c1) to head (bd68572).
Report is 41 commits behind head on main.

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Flag	BASE (10985c1)	HEAD (bd68572)
unittests	3	2

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #34       +/-   ##
===========================================
- Coverage   72.81%   49.37%   -23.45%     
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  Files           3        3               
  Lines         103      160       +57     
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+ Hits           75       79        +4     
- Misses         28       81       +53     

Flag	Coverage Δ
unittests	49.37% <ø> (-23.45%)	⬇️

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[jdebacker]

@john-p-ryan Thanks for this PR! LGTM, merging.