raised error - Githubissues

Description

This PR adds a Value Error that is raised if the monotonicity is anything but None in the Linear model

Closes #(replace this parenthetical with issue number)

N/A

[x] The code uses informative and accurate variable and function names
[ ] The functionality is factored out into functions and methods with logical interfaces
[x] Comments are up to date, document intent, and there are no commented-out code blocks
[x] Commenting and/or documentation is sufficient for others to be able to understand intent and implementation
[x] TODOs have been eliminated from the code
[x] The corresponding issue number (e.g. #278) has been searched for in the code to find relevant notes
[ ] Documentation has been redeployed