Nexus: Add Density functionality to qdens tool

[mcbennet]

Proposed changes

This PR extends the tool qdens to allow the user to read Density data within stat.h5.

What type(s) of changes does this code introduce?

• New feature

Does this introduce a breaking change?

• No

What systems has this change been tested on?

Linux laptop

Checklist

• Yes. This PR is up to date with current the current state of 'develop'
• N/A. Code added or changed in the PR has been clang-formatted
• No. This PR adds tests to cover any new code, or to catch a bug that is being fixed
• N/A. Documentation has been added (if appropriate)

[qmc-robot]

Can one of the admins verify this patch?

[jtkrogel]

nxs-test for qdens passes? Is twist averaging supported or not? If not, need to at least raise an error.

[prckent]

qdens test passes:

      Start 1725: ntest_nexus_qmca
59/69 Test #1725: ntest_nexus_qmca ..............................   Passed    7.15 sec
      Start 1726: ntest_nexus_qmc_fit
60/69 Test #1726: ntest_nexus_qmc_fit ...........................   Passed    1.19 sec
      Start 1727: ntest_nexus_qdens
61/69 Test #1727: ntest_nexus_qdens .............................   Passed    1.23 sec
      Start 1728: ntest_nexus_example_gamess_H2O
62/69 Test #1728: ntest_nexus_example_gamess_H2O ................   Passed    3.74 sec
      Start 1729: ntest_nexus_example_pwscf_relax_Ge_T
63/69 Test #1729: ntest_nexus_example_pwscf_relax_Ge_T ..........   Passed    2.17 sec

[mcbennet]

Yes, I saw the same. Thank you @prckent . I am working to confirm that averaging is supported.

[mcbennet]

The tool does not succeed when reading Density data from a periodic system -- even with a single twist.
Will follow-up with a fix for this as well as added support for averaging.

[mcbennet]

This should be ready to go with twist averaging now supported. qdens still passes nxs-test.

[ye-luo]

Test this please