Add option to only use multiple-bigm on bound constraints, normal bigm on everything else

[emma58]

Fixes # .

Summary/Motivation:

Though multiple-bigm is at least as tight as traditional bigm, it's not always computationally advantageous. In particular, there are cases where using multiple bigm only on the bound constraints (constraints of the form l <= x <= u) and traditional bigm elsewhere in the model is the fastest to solve. This adds an option to the gdp.mbigm transformation to do this.

Changes proposed in this PR:

• A little big of rearranging in the bigm transformation to make the part of the code that builds the constraint expressions modular
• Adds only_mbigm_bound_constraints option to mbigm transformation. (Note that this requires reduce_bound_constraints to be True as well (which it is by default))

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