Remove NUMBER_TO_ELEMENTS

[IAlibay]

Fixes #2699

Changes made in this Pull Request:

• Removes MDAnalysis.topology._elements.py
• Switches NUMBER_TO_ELEMENTS to Z2SYMB in TOPParser
• Adds a more thorough elements test and starts cleaning up tests in anticipation of Removing guess by default in TOPParser #2651

Additional context:

Unfortunately it seems I tackled this the wrong way around. The elements testing for TOPParser is a little bit all over the place at the moment, but I plan to more thoroughly change things with #2651, if anything, the test_elements function for PRM12 is going to get moved to TOPBase.

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #2700 into develop will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2700      +/-   ##
===========================================
- Coverage    91.15%   91.12%   -0.03%     
===========================================
  Files          175      174       -1     
  Lines        23719    23650      -69     
  Branches      3104     3104              
===========================================
- Hits         21620    21551      -69     
  Misses        1480     1480              
  Partials       619      619              

Impacted Files	Coverage Δ
package/MDAnalysis/topology/tables.py	100.00% <ø> (ø)
package/MDAnalysis/topology/TOPParser.py	100.00% <100.00%> (ø)
auxiliary/base.py	90.84% <0.00%> (-0.56%)	⬇️
coordinates/base.py	94.67% <0.00%> (-0.29%)	⬇️
util.py	88.19% <0.00%> (-0.09%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 55afb4c...4019011. Read the comment docs.

[orbeckst]

Just make the CHANGELOG message more informative but overall LGTM.