Add model to validate the placement block

[woodtp]

Adds a PlacementParameters model according to the standard and incorporates it into the BaseElement.

PlacementParameters defines four fields: offset, to_point, base_item, and from_point. A ValidationError is raised when:

• offset is not a finite number, such as a string, inf or nan
• to_point or from_point is not one of ENTRANCE_END, CENTER, EXIT_END, or ZERO_POINT.

I placed the corresponding tests in separate classes to make it easier to run only the tests relevant to these additions.