Remove Margins table and disable getMarginsImpacts from multi-regional models

[WesIngwersen]

While a two-region model are successfully create with a Margins table, this is just loading the Margins table from the national IO and applying it equally to both regions in the model. This is not throwing an error but it is creating a imbalance and was not intended.

Recommend remove Margins table from these models (only call it for single region models based on BEA IO for now) and return an error message if getMarginsImpacts is called for these models.