Reports - make getTemplateFile & compileContent functions over-ridable, default to form.tpl

This fix makes it possible to override parts of the endProcess function without over-riding the whole thing - specifically the steps of choosing the template & compiling the content. Template will also default to Form.tpl if one doesn't match the extension.

Usage examples include using a different template for a printed report (as civiengage does), or adding / tweaking content in reports that extend this class