Allow most reports to include searchable custom fields as Display Columns and Filters

Modify the PHP class files for all reports (except possibly Case reports) that don't already support custom fields to enable custom fields for the appropriate record type(s) by defining the $_customGroupExtends array.

• For 'Contact' reports, use the following array:
  protected $_customGroupExtends = array( 'Contact', 'Individual', 'Household', 'Organization' );
• For component reports (other than Case), include custom groups which extends the primary object being reported on:
  EXAMPLE: Contribution reports - protected $_customGroupExtends = array( 'Contribution' );

We'll need to consult with CiviCase users / implementors (Dave D et al) to see if Case reports should be extended, and if so which objects. Ping me (dgg) if you questions on specific reports on which object(s) to include in the extends array.