Improve packets order interface

[felixblanke]

• Makes WaveletPacket.get_level static
• Adds Option for WaveletPacket.get_level to return the 1d natural order.
• Adds WaveletPacket2D.get_level to have a consistent interface (with our 1d case as well as pywt)
• Adds unit tests for both 1d orders and 2d natural order

[v0lta]

Thanks, I like this PR. It introduces pywt compatible interfaces, which will make it easier to switch. I merged the main, and so far, all tests have checked out.

[v0lta]

This PR is ready to be merged.