Add a module physics.observables that can be used to calculate observables.
Currently only "local" observables are implemented, that is, observables that can be defined by
a single (matrix) value M_i for each site i, in addition to the Hamiltonian.
This covers common cases, including what we call "generalized charge", which includes:
- probability density
- charge / charge density
- spin / spin density
also it includes the "generalized currents" associated with the generalized charge:
- probability current
- charge current
- spin current
and finally "source" terms, the most prominent example of which is:
- spin torque
This supersedes the current-calculating part of !17 (closed). This closes #14 (closed).