Backward compatibility for Accord Project Templates v0.6

[jeromesimeon]

There should be a way for the new version of Cicero (implementing v0.7 of the accord project template specification) to execute older templates (implementing v0.6 of the accord project template specification).

[jeromesimeon]

There are a couple of patches in #113 which should be replaced by a more robust/well tested approach.

[jeromesimeon]

Probably the execute call should detect whether it's running a v0.7 or v0.6 template https://github.com/accordproject/cicero/blob/master/packages/cicero-engine/lib/engine.js#L137

[jeromesimeon]

@mttrbrts @dselman feedback?

[jeromesimeon]

There was some follow up cleaning of the corresponding code. We won't support backward compatibility within the same Cicero engine. Closing.