Second order xxz

[shangtai]

Introducing second order product formula to XXZ then M steps of the Trotter-Suzuki decomposition for qubits.

[marekgluza]

@shangtai can you please

• [x] consolidate the functions by adding an argument order=1
• [x] add documentation
• [x] let me know by when will it be delivered? Ideal would be today

Thanks!

[marekgluza]

merging but there are bugs that need to be tested