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

1.29 ms ± 12.9 μs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

with this branch and

2.78 ms ± 20.4 μs per loop (mean ± std. dev. of 7 runs, 100 loops each)

with the current main branch.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.08%. Comparing base (a9cebee) to head (35aee00).
⚠️ Report is 97 commits behind head on main.

Files with missing lines	Patch %	Lines
cf_xarray/helpers.py	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
+ Coverage   85.78%   87.08%   +1.29%     
==========================================
  Files          13       15       +2     
  Lines        2364     3220     +856     
  Branches      183      307     +124     
==========================================
+ Hits         2028     2804     +776     
- Misses        303      371      +68     
- Partials       33       45      +12     

Flag	Coverage Δ
mypy	46.00% <0.00%> (+7.46%)	⬆️
unittests	92.84% <100.00%> (-1.15%)	⬇️

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