Created by: gcarleo
This PR adds the possibility to have a constant shift in LocalOperator, such that
in general one can have LocalOp = \sum_i O_i + constant.
Algebraic operations on LocalOperator have been updated accordingly. This also removes the ambiguity when constructing a LocalOperator without arguments.
This PR also fixes a few issues in the QuantumStateReconstruction application, mostly existing because of the copy constructor in LocalOperator introduced in #117. This has now been removed because intrinsically unsafe (we can't easily copy Hilbert objects) and was most likely giving rise to some issues when creating vectors of LocalOperator, as needed by QuantumStateReconstruction.