Add the fmt_units() method

[rich-iannone]

This PR adds the fmt_units() method. This performs a conversion of units in the units notation syntax (e.g., "x10^9 / L" or "x10^9 L^-1", etc.) to HTML. The method, like others of the fmt_*(), only concerns itself with text transformations in the table body. Other methods will also gain the ability to convert text in units notation to nicely formatted HTML in later PRs.

Here's an example that uses fmt_units() with the illness dataset. It so happens that the units column has strings in units notation, so, we just need to point this method to that column:

from great_tables import GT, style, loc
from great_tables.data import illness

(
    GT(illness, rowname_col="test")
    .fmt_units(columns="units")
    .fmt_number(columns=lambda x: x.startswith("day"), decimals=2, drop_trailing_zeros=True)
    .tab_header(title="Laboratory Findings for the YF Patient")
    .tab_spanner(label="Day", columns=lambda x: x.startswith("day"))
    .tab_spanner(label="Normal Range", columns=lambda x: x.startswith("norm"))
    .cols_label(
      norm_l="Lower",
      norm_u="Upper",
      units="Units"
    )
    .opt_vertical_padding(scale=0.4)
    .opt_align_table_header(align="left")
    .tab_options(heading_padding="10px")
    .tab_style(
        locations=loc.body(columns="norm_l"),
        style=style.borders(sides="left")
    )
    .opt_vertical_padding(scale=0.5)
)

Fixes: #211
Partially addresses the .epic issue: #169

[machow]

Shoot sorry---I thought this PR had re-requested review, so made some comments 😅 . I'm going to submit the review, in case they're helpful, but sorry for jumping early on this :o

[machow]

It's not clear what about fmt units is being tested here. Please...

• test values to be formatted one by one
• only test values that test / make clear something new about fmt_units

[rich-iannone]

This is now improved. Thanks!

[machow]

Thanks for breaking this up. Do you mind expressing this using pytest.mark.parametrize, so that each is an individual test? This way, the CI will show at a glance which piece is failing, and we see the inputs / outputs close to eachother. (seems reasonable to keep the comments between each parameter case)

Currently, you have to look down at the specific assert, and then up to match the output with the input

[rich-iannone]

Now done.