Remove mutable default arguments

[guyer]

As documented (under "Default parameter values are evaluated from left to right when the function definition is executed"), using mutables as default arguments can have unexpected results.

This is done in most of the Viewer definitions (limits={}), as well as a handful of other cases.

As documented:

A way around this is to use None as the default, and explicitly test for it in the body of the function

See also https://stackoverflow.com/questions/26320899/why-is-the-empty-dictionary-a-dangerous-default-value-in-python.