Fix inconsistency in sympy symbols in Models

There are multiple things here, might want to break this up into multiple MR's.

• Fix #15 (closed) by removing copy(key) in Model.

• Fix a related error in the kwant.qsymm tests by removing the real=True flag in hamiltonian_from_family.

• Refactor bloch_family to avoid unnecessary conversion between Model and BlochModel (as it turns out this had nothing to do with the bugs, but make the code nicer).