Implemented formatting of floats when printing Expressions

[kjartan-at-bravos]

Usage:

>>> x = Variable(pl.DataFrame({"t": [1,2]})
>>> y = 1.234 * x
>>> printy.to_str(float_format="%0.1f")
[1]: 1.2 x1
[2]: 1.2 x2

The most important use case is when comparing the string representation of Expressions with expected representations in unit tests.

[codecov]

Codecov Report

Attention: Patch coverage is 73.17073% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 93.03%. Comparing base (235103b) to head (dcb2dac).

❗ Current head dcb2dac differs from pull request most recent head 0455a56. Consider uploading reports for the commit 0455a56 to get more accurate results

Files	Patch %	Lines
src/pyoframe/util.py	71.05%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   94.12%   93.03%   -1.10%     
==========================================
  Files          13       13              
  Lines         766      804      +38     
==========================================
+ Hits          721      748      +27     
- Misses         45       56      +11     

Flag	Coverage Δ
smart-tests	93.03% <73.17%> (-1.10%)	⬇️

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