Automatic selection of quadrature source for methods with nonzero degree

[ablech]

Currently, to select a quadrature method, the geometry, type and source need to be specified (in addition to size or degree).

In general, users might more be more interested to select the method with the highest efficiency than selecting a method from a specific source. In this case, it would be advantagous to only specify the geometry and the type and let openquad select the source.

This is necessary without changes on the interface, since we already use the form 'geometey-type-source'. I would suggest to make '-source' optional and add automatic selection of the source, if omitted.

The same strategy could also be applied to automatic selection of the type. This is useful, if a user is only interested in the highest efficiency for a given degree. However, this might not be a general usecase (unless combined with requesting additional properties, see #4) since the different types have different convergence properties.