add a round_to kwarg in all the plot methods

[drbenvincent]

Right now the plot methods report numerical results with a hard-wired number of decimal places. The user should be able to specify this. Add a round_to (inspired by what arviz uses) kwarg to all the plot methods to achieve this.

• Add the round_to kwarg and associated utility functions
• The docstrings will need to be updated.
• Add tests to cover the new functions