v0.3.0

--Generalized extension name into the abstract type ExtendGPU to allow for alternative GPU-based methods.
--Added manual garbage collection to reduce time spent on GPU GC.
--Corrected variable name generation when variables with indices are used (e.g. x[1:5] instead of x).
--Corrected recognition of variables with indices in some edge cases.
--Corrected mid_expr, which sometimes returned results inconsistent with McCormick.jl.
--Added a ::Nothing relaxation rule.
--Corrected some typos in multiplication relaxation rules.
--Added "cut" operator to multiplication and division operations.
--Added min/max rules (may need further development/verification).
--Changed "factor" to no longer modify input expressions.
--Added some support for "constants", which are inputs that should be treated symbolically but should not be expanded into McCormick tuples.
--Added "sort_vars" function, which groups McCormick tuples together, and places constants at the front, before alphabetically sorting McCormick tuple groups.
--Reordered user-facing inputs and outputs to [cv, cc, lo, hi] instead of [lo, hi, cv, cc].