SpinDensityEstimator needs to be upgraded to support open boundary conditions

[prckent]

Is your feature request related to a problem? Please describe.

The SpinDensityEstimator currently does not support open boundary conditions. Once supported we can deprecate or remove the DensityEstimator since the SpinDensityEstimator has a superset of the DensityEstimator functionality.

Describe the solution you'd like

Implement open boundary conditions

Additional context

This would be a good first code contribution or similarly a good topic for someone less familiar with the simulation/physics aspects of the code. The code in DensityEstimator should be translatable to SpinDensityEstimator.

See also https://github.com/QMCPACK/qmcpack/issues/2620