[FEAT] fill_nan and not_nan expressions

[colin-ho]

Adds expressions for fill_nan and not_nan

[codecov]

Codecov Report

Attention: Patch coverage is 80.34188% with 23 lines in your changes missing coverage. Please review.

Project coverage is 63.51%. Comparing base (f99fcd8) to head (a2fddef).
Report is 2 commits behind head on main.

❗ Current head a2fddef differs from pull request most recent head 0d8ad16

Please upload reports for the commit 0d8ad16 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2313      +/-   ##
==========================================
+ Coverage   63.48%   63.51%   +0.02%     
==========================================
  Files         906      908       +2     
  Lines      101891   102008     +117     
==========================================
+ Hits        64687    64786      +99     
- Misses      37204    37222      +18     

Files	Coverage Δ
daft/expressions/expressions.py	93.09% <100.00%> (+0.07%)	⬆️
src/daft-core/src/array/ops/float.rs	100.00% <100.00%> (ø)
src/daft-core/src/python/series.rs	94.52% <100.00%> (+0.06%)	⬆️
src/daft-dsl/src/functions/float/mod.rs	100.00% <100.00%> (ø)
src/daft-dsl/src/python.rs	91.82% <100.00%> (+0.10%)	⬆️
daft/series.py	91.83% <85.71%> (-0.08%)	⬇️
src/daft-core/src/series/ops/float.rs	76.92% <85.71%> (+10.25%)	⬆️
src/daft-dsl/src/functions/float/not_nan.rs	64.00% <64.00%> (ø)
src/daft-dsl/src/functions/float/fill_nan.rs	53.84% <53.84%> (ø)

... and 6 files with indirect coverage changes