[Example] Add comet.py and fix to_numpy() on sparse matrix fields

[archibate]

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Have you seen the NEOWISE comet yet 😆

[codecov]

Codecov Report

Merging #1583 into master will increase coverage by 0.04%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #1583      +/-   ##
==========================================
+ Coverage   67.47%   67.52%   +0.04%     
==========================================
  Files          40       40              
  Lines        5624     5632       +8     
  Branches      982      982              
==========================================
+ Hits         3795     3803       +8     
  Misses       1661     1661              
  Partials      168      168              

Impacted Files	Coverage Δ
python/taichi/lang/matrix.py	91.90% <66.66%> (-0.13%)	⬇️
python/taichi/lang/__init__.py	80.20% <100.00%> (-0.01%)	⬇️
python/taichi/lang/ops.py	93.33% <0.00%> (+0.81%)	⬆️

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[archibate]

Can we automatically use ti.no_activate(x) when:

1. the following struct-for is on x.
2. within that struct-for, only x[i] is written.

e.g.:

for i in x:
  x[i] = 1  # OK to optimize
  x[i // 2] = 1  # won't optimize

Thoughts? @xumingkuan @yuanming-hu

[archibate]

Merging now given that this is a trivial change.