Allow building and solving models iteratively

[coroa]

Perhaps with some sort of callback to update some parameters/variables.

[fneum]

To set warmstart of a model with solution of previous iteration:

set_start_value.(all_variables(model), value.(all_variables(model)))

To set a new coefficient in a model (e.g. changed line impedance):

@constraint(model, con, 2x <= 1)
set_coefficient(con, x, 3)