Updated constructors for mesh info classes to conform with other filters

The CellMeasures and MeshQuality filters had constructors that took the metric that the filter should generate. However, this is different than the iterface of the rest of the filters. To make the interface more consistent, these filters now have a default (no argument) constructor, and the metric to compute is selected via a method. This makes it more clear what is being done.

In addition, the documentation for these two classes is updated.