Add default values for bout_tdim, bout_xdim, bout_ydim, bout_zdim, so that they always exist

[johnomotani]

Some methods need metadata["bout_ydim"], etc., and it can be useful to have these set even when no geometry is applied.

[codecov-io]

Codecov Report

Merging #179 (756d73e) into master (f213223) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
+ Coverage   75.01%   75.04%   +0.03%     
==========================================
  Files          12       12              
  Lines        2065     2068       +3     
  Branches      475      475              
==========================================
+ Hits         1549     1552       +3     
  Misses        347      347              
  Partials      169      169              

Impacted Files	Coverage Δ
xbout/geometries.py	79.45% <100.00%> (+0.43%)	⬆️

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[johnomotani]

Closing this as I am not totally sure it is correct without an extra fix to default values in _set_default_toroidal_coordinates() that's present in #180 (which includes this commit already).