SeaPy is now a Python3 module instead of Python2

- Update .gitignore to ignore _build

- Support enabling/disabling any SEA object.

It is now possible to enable/disable an SEA object. The obj.available
flag indicates whether the object is available and will be included in
the analysis. Use _DEPENDENCIES to set which objects the availability of the object depends on.

- Lots of work on Connections and couplings
- Couplings are updated when components are added to/removed from connection.

- Add missing angular property to NewFrequency

- New FilterList class is a list capable of filtering on item attributes.

The list has a filter method, which can be used to filter the list on
items for which the specified attribute and value evaluates to True.
Most lists have been replaced by FilterList