Add FunctionView class

[Prof Garth Wells]

Having a FunctionView for sub-function views would make clearer what's happenning in program and will simplify code in Function.

It will resolve Issue #194.

See thread at https://www.mail-archive.com/fenics%40fenicsproject.org/msg02398.html

[Anders Logg (Chalmers)]

Removing milestone: 1.7 (automated comment)

[Jan Blechta]

Eventually FunctionSpaceView class too. See also #918.