Merge pull request #5 from felipeacsi/master

Typos
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Most lists have been replaced by FilterList
Changing from blockdiag to png images