Calculate better y-axis limits to use for Bode and noise plots; change default phase y-axis tick steps to 45/15 degrees.

(BREAKING) Change grid zorder config key to a grid section, and add settings for transparency; display minor grid.

When deepcopying a circuit, the nodes were also deepcopied. This is a problem because
nodes are supposed to be singletons. When the user wanted to analyse a deepcopied
circuit that had previously been analysed, the input component would be added with
different nodes even though those nodes would exist in the circuit already (same
name, but different memory locations), leading to singular matrices when solving.

The fix is to implement special deepcopy behaviour for circuits using __deepcopy__.
This is not ideal; it might be worth removing the singleton property of nodes and
make them circuit-specific.