[Feature]: Mid-run population injections and other tools

[Muhammad-Ayan-No1]

Feature Request

Using pysr i wanted to automate some task however it requires injection of new population (i get the current population apply some custom fns to it and inject back) I also require training Data to be changed midway etc

All this is required because my training data has values with each value being {value:Num, err:SomeNum} because its a physical measurment it has err the + - error rate idk and i dont thing pysr lets you say hey heres my data just remember its valid of N = n +- a so like it trys to use different values of possible N of each data item such that the equation stays clean across all other data points

[MilesCranmer]

The new "guesses" feature should allow this in #999