Explicitly prevent the instantiation of abstract classes

[alihamdan]

Closes #188

Inheriting from ABC (or using ABCMeta as a metaclass) prevents the instantiation of classes that have abstract methods but not classes without abstract methods. This PR improves the error in case someone tries to instantiate Element, AbstractLoad, or AbstractBranch. I used the same error type and message used by ABC. An example before and after:

Before
could raise any error; currently raises:

>>> lf.AbstractLoad("load", bus=lf.Bus("bus", phases="abcn"), powers=[-1e3, -1e3, -1e3])
AttributeError: 'AbstractLoad' object has no attribute 'type'

After
always raises

>>> lf.AbstractLoad("load", bus=lf.Bus("bus", phases="abcn"), powers=[-1e3, -1e3, -1e3])
TypeError: Can't instantiate abstract class AbstractLoad