Create ElementBackend encapsulating tasks related to a finite element

[kynan]

Properties:

• q number of quadrature points
• d (topological) degree
• v number of vertices
• e the corresponding ufl.FiniteElement

Responsibilities:

• query quadrature and shape information (from femtools or FIAT)
• generate transform to physical kernel using a backend-specific template

[kynan]

This will probably not be needed any more because these tasks will be taken care of by Transformation and FormBackend