Replace `MutableData` with `Data` in the `PropensityScore` class

[drbenvincent]

FutureWarning: MutableData is deprecated. All Data variables are now mutable. Use Data instead.

[klaushajdaraj]

Hi @drbenvincent , could I take this one? :) Thank you in advance!

[drbenvincent]

Hi @klaushajdaraj
Sure, I'll assign you to this issue.

[drbenvincent]

Closed by #455