improve xchr function

## Goal
improve xchr function and address problems in #199 #216 #217 

## Proposed approach
1. Figure out the issue with the simple example
2. Modify penetrance function

## Scope
AlphaPeel/example
AlphaPeel/src/tinypeel
Still not sure if we should do something on tinyhouse 

## Done when
Open 3 Mar 2026, it will be completed in one month