Document a workaround for conflicts in open_boutdataset(); some tidying

[johnomotani]

• Add troubleshooting section to open_boutdataset docstring. Says what to do if variables conflict when combining data from several restarts.
• Tidying up of other docstrings
• Refactor regions property to _regions, ensure it is being used correctly - previously I think attrs["regions"] was (accidentally) being used through the xarray feature of making things attrs accessible with .foo syntax.

[codecov-commenter]

Codecov Report

Merging #217 (7a1093f) into master (22e1589) will increase coverage by 0.03%. The diff coverage is 80.39%.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   75.34%   75.37%   +0.03%     
==========================================
  Files          14       14              
  Lines        2644     2644              
  Branches      622      622              
==========================================
+ Hits         1992     1993       +1     
+ Misses        423      421       -2     
- Partials      229      230       +1     

Impacted Files	Coverage Δ
xbout/region.py	85.39% <70.83%> (ø)
xbout/utils.py	83.03% <81.81%> (ø)
xbout/boutdataarray.py	77.14% <88.88%> (+0.28%)	:arrow_up:
xbout/boutdataset.py	75.60% <100.00%> (ø)
xbout/load.py	78.15% <100.00%> (ø)
xbout/plotting/plotfuncs.py	48.27% <100.00%> (ø)
xbout/plotting/utils.py	75.24% <100.00%> (ø)

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