Support P-Split Reformulation

[dnguyen227]

This PR adds the P-Split reformulation technique. The technique is called with a vector of partitions:
For example for two partitions (p=2):

model = GDPModel()
...
@variable(model, x[1:4], Disjunct(...))
...
partition = [ [x[1] , x[2]], [x[3] , x[4]]] #one partition contains x1,x2 and the other contains x3,x4
optimize!(model, gdp_method = PSplit(partition))

In src/psplit.jl are extensions for reformulate_disjunct_constraint as well as new functions for building partitioned expressions.