feat(python): Add top-level nth(n) method, to go with existing first and last

[alexander-beedie]

Had a local use-case that would have benefited from this and realised we hadn't exposed the existing Expr::nth (as used by first & last) up to the Python API. Allows for selection of the "nth" col or value, depending on context. Also slightly clarified the existing first/last docstrings.

Example

import polars as pl

df = pl.DataFrame({
    "a": [1, 8, 3],
    "b": [4.5, 5.0, 2.5],
    "c": ["foo", "bar", "baz"],
})
# shape: (3, 3)
# ┌─────┬─────┬─────┐
# │ a   ┆ b   ┆ c   │
# │ --- ┆ --- ┆ --- │
# │ i64 ┆ f64 ┆ str │
# ╞═════╪═════╪═════╡
# │ 1   ┆ 4.5 ┆ foo │
# │ 8   ┆ 5.0 ┆ bar │
# │ 3   ┆ 2.5 ┆ baz │
# └─────┴─────┴─────┘

df.select(pl.nth(1))
# shape: (3, 1)
# ┌─────┐
# │ b   │
# │ --- │
# │ f64 │
# ╞═════╡
# │ 4.5 │
# │ 5.0 │
# │ 2.5 │
# └─────┘

df.select(pl.nth(1,"c","b"))
# shape: (1, 2)
# ┌─────┬─────┐
# │ c   ┆ b   │
# │ --- ┆ --- │
# │ str ┆ f64 │
# ╞═════╪═════╡
# │ bar ┆ 5.0 │
# └─────┴─────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.97%. Comparing base (acb601d) to head (29165e4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16112   +/-   ##
=======================================
  Coverage   80.97%   80.97%           
=======================================
  Files        1386     1386           
  Lines      178479   178491   +12     
  Branches     2877     2878    +1     
=======================================
+ Hits       144530   144541   +11     
  Misses      33458    33458           
- Partials      491      492    +1     

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[ritchie46]

Funny, didn't realize this was catching dust.