Resolve errors in LaTeX printer for set operations / multidimensional sets

[jsiirola]

Fixes # .

Summary/Motivation:

This resolves errors in the latex_printer() for models with set operators (in particular, SetProduct indexing sets) and multidinensional sets.

Changes proposed in this PR:

• minor performance improvements in internal lookups
• resolve errors naming IndexTemplate objects for multidimensionsl sets
• resolve overlapping placeholder IDs for models with > 10 vars or params
• pull out some embedded constants (make it easier to override)
• Improved output of anonymous set expressions
• add test of pmedian (which motivated these changes)

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[mrmundt]

Does this do anything to address issues reported in #3121 ?

[mrmundt]

NEOS is having some issues today, it appears.

[jsiirola]

Does this do anything to address issues reported in #3121 ?

Nope: #3121 is asking for support of block structured models That will be a separate PR (this is addressing indexing issues.

[jsiirola]

Coverage failure is a codecov issue: the latex_printer is covered at over 92%,