Derived templates in TemplateBody instead of TemplateInstance

The BNF in the TTCN-3 standard states that the "modifies" keyword and the derived reference (i.e. the base template) are part of the TemplateBody rule and not the TemplateInstance rule (as it is in Titan currently).

This means that derived templates should be allowed everywhere a template is allowed, and not just everywhere a template instance (i.e. in-line-template) is allowed.

/cc @aknappqwt @mmagyari