Parameterise the betaval range of Linear Epsilon constraint

[asiripanich]

Any reason why we hard-coded betatree range here? instead of making it configurable? maybe it can be something like 0.0:(1/mdata.pointsperdim):1.0.

https://github.com/anriseth/MultiJuMP.jl/blob/0921721035793896798b457ee184d4ffa0b4e867/src/linear.jl#L107-L109