BUG: check that `_var_index` exists in the utility methods

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of eeco.

Description

There are numerous instances of model._var_index being accessed in utils.py without checking that we have created that attribute. Since _var_index is automatically created within EECO and not intended to be made by the user, it would be good to ensure we check for its existence along any code paths where it may have not been created.

Further discussion of this issue can be found in this PR thread: https://github.com/we3lab/eeco/pull/50/changes#r3786497573

Expected Behavior

If _var_index does not already exist, then it should be created with the built-in method create_pyomo_model_index_ref.

If _var_index does exist, then it should be left untouched.

Reproduction

There are numerous ways to create an AttributeError. For example, calling utils.multiply() with a model object that does not have the attribute _var_index.

Start in utils.py by locating the utility methods that access model._var_index, then read create_pyomo_model_index_ref and the PR discussion linked in the issue. Exercise a method such as utils.multiply with a model lacking _var_index, and verify that missing attributes are created while existing _var_index values remain untouched.