Duals (undual, Hodge dual, PGA dual) and their implementations

[brainandforce]

There's a lot of different duals available in geometric algebra as seen in the issue title, and I want to make sure that they are implemented correctly and clearly distinguished.

[brainandforce]

Updating this to note that the left and right complements (which are metric-independent duals) were added in v0.1.3. Still needing implementation are the Hodge dual and undual.