Optimize add_bounds

[bouweandela]

Simplify the code for adding bounds by avoiding adding and removing indexes. This code is also a bit faster than the original.

In [1]: import xarray as xr, cf_xarray

In [2]: airds = xr.tutorial.open_dataset("air_temperature")

In [3]: %timeit airds.cf.add_bounds("time")

results in

2.05 ms ± 37 μs per loop (mean ± std. dev. of 7 runs, 100 loops each)

with this branch and

3.36 ms ± 318 μs per loop (mean ± std. dev. of 7 runs, 100 loops each)

with the current main branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.80%. Comparing base (a9cebee) to head (f466765).
⚠️ Report is 97 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
+ Coverage   85.78%   92.80%   +7.01%     
==========================================
  Files          13       14       +1     
  Lines        2364     2403      +39     
  Branches      183        0     -183     
==========================================
+ Hits         2028     2230     +202     
+ Misses        303      173     -130     
+ Partials       33        0      -33     

Flag	Coverage Δ
mypy	?
unittests	92.80% <100.00%> (-1.19%)	⬇️

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