Added Primary_Contact__c formula field on Contact

# Warning

# Info
* A new formula field, Contact.npsp__Primary_Contact__c, now provides a read only checkbox which is checked if the Contact is the Primary Contact on their Account.  This can be used in report criteria to build a deduplicated list of all Primary Contacts for Households.  Since the field is read only, it is not added to any layouts by default but is available for use in reports.

# Issues

Fixes #971