Add Summarizing Printing Methods

[pulsipher]

There is no way to have GDPModels print any information pertaining to GDPData automatically since GDPModels are just JuMP.Models. However, we can create our own method(s) (e.g., print_gdp_model, latex_gdp_model, and summarize_gdp_model) to properly print a GDPModel by taking GDPData and the current reformulation state into account.