Obtaining basic metadata from dimod models

**Application**
Dimod-supported characterization the object properties

**Proposed Solution**
For each of the canonical models, eg. BQM, DQM, there could be a method that puts together some of the model properties, ie. num_variables, num_biases, num_interactions, etc. and returns them as a dict.

**Alternatives Considered**
The alternative would be to look for all the attributes and methods available and build your own version. This has the danger of breaking compatibility as these models get updated.

**Additional Context**
This is important for analyzing performance of solvers based on problem sizes and other basic properties.