Open boundary conditions spindensity estimator crashes

**Describe the bug**
https://cdash.qmcpack.org/CDash/testDetails.php?test=9252705&build=132150

This test fails stating that the estimator is not yet implemented. Can we implement it simply, or should the test be modified?
I am asking this question in the spirit of getting the nightlies to "green" on cdash.

**To Reproduce**

ctest -R short-LiH_dimer_pp-vmc_hf_sdj_estimator_spindensity

**Expected behavior**
Test passes

**System:**
nitrogen and sulfur.

**Additional context**

Start by running ctest -R short-LiH_dimer_pp-vmc_hf_sdj_estimator_spindensity and inspect the linked CDash failure. Determine whether the missing spindensity estimator should be implemented or the test changed; done means the test passes and the chosen resolution is documented.

Written by the indexing model from the issue text.