Refactor visualization module

[matulni]

Graphix module visualization.py is currently very convoluted and does not exploit the new flow API.
Among other issues, the flow computation and plot generation logic are largely intertwined in method Pattern.draw.

Note:

• When a pattern has flow, Pattern.draw plots the pattern's flow + (optionally) Clifford commands.
• When a pattern does not have flow Pattern.draw plots the pattern's XZ-corrections + (optionally) Clifford commands.

I propose to implement new draw methods for Flow and XZCorrections objects and call them from Pattern.draw where appropriate.
To be discussed how to allow for Clifford commands plotting.

Related: #439, #444