w_multi_west ibstates fix

[jeremyleung521]

Issue Number. Is this pull request related to any outstanding issues? If so, list the issue number.

Describe the changes made. A clear and concise description of what the problem is and what you did to fix it. E.g. [...] was happening and I've changed [...] to fix it.
There were two issues with w_multi_west's --ibstates option which
a) prevented w_trace from working properly and
b) combined the istate_pcoord incorrectly.

a) is due to a broken link/object reference in the ibstates/index dataset, which lead to a failure when attempting to retrieve the ibstates dataset using that link.
b) is due to the fact that I forgot to specify axis=0 when appending the pcoord arrays.

Goals and Outstanding Issues. A clear and concise list of goals (to be) accomplished.

• Allow w_trace to run on files combined with w_multi_west
• Combine istates_pcoord dataset correctly

Major files changed.

• src/westpa/cli/tools/w_multi_west.py

Status.

• Bug fix (non-breaking change which fixes an issue)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.

Additional context. Add any other context or screenshots about the pull request here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.11%. Comparing base (6013164) to head (51554c3).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #387      +/-   ##
===========================================
- Coverage    42.18%   42.11%   -0.07%     
===========================================
  Files          131      131              
  Lines        17022    17005      -17     
===========================================
- Hits          7180     7161      -19     
- Misses        9842     9844       +2     

Flag	Coverage Δ
unittests	42.11% <100.00%> (-0.07%)	⬇️

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[darianyang]

👍