Allow set_onsite_coupling! with zero

[kbarros]

This fixes #110 and performs some additional reorganization.

[kbarros]

The proposed new behavior is:

sys = System(...)
S = spin_operators(sys, 1)
set_onsite_coupling!(sys, zero(S[3]), 1) # does nothing
set_onsite_coupling!(sys, S[3]^2, 1)     # sets a coupling
set_onsite_coupling!(sys, zero(S[3]), 1) # emits a warning and removes coupling
set_onsite_coupling!(sys, S[3]^2, 1)     # sets a coupling *without warning* (does not detect existing coupling)