Make sure we use MIME when calling repr in GroupedDataFrame printing

[bkamins]

What this PR changes:

1. better code organization (so that later changes to DataFrameRow and GroupedDataFrame printing are needed to be made in one file).
2. key of the PR: use MIME("text/plain) when calling repr to show group label key. This is needed to avoid printing type name in some cases (which is not useful).

This resolves the issue that before this change docs building was not passing due to the changes in InlineStrings.jl printing rules.

[nalimilan]

Looks good, but can you just detail in what case printing is changed for reference?

[nalimilan]

Why not use a triple-quoted string instead?

[ronisbr]

I was about to write the same comment! Something like that:

"""
GroupedDataFrame with 3 groups based on key: id
First Group (1 row): id = "a"
 Row │ id       value
     │ String1  Int64
─────┼────────────────
   1 │ a            1
⋮
Last Group (1 row): id = "c"
 Row │ id       value
     │ String1  Int64
─────┼────────────────
   1 │ c            3
"""

[bkamins]

I will re-check it (this is what I did initially, but I was getting test errors constantly for some reason I did not understand)

[bkamins]

OK - I have managed to find problems. The issues were trailing whitespace in some cases.

[bkamins]

What we currently have is:

julia> using DataFrames

julia> using InlineStrings

julia> df = DataFrame(id=inlinestrings(["a", "b"]), x=1:2)
2×2 DataFrame
 Row │ id       x
     │ String1  Int64
─────┼────────────────
   1 │ a            1
   2 │ b            2

julia> groupby(df, :id)
GroupedDataFrame with 2 groups based on key: id
First Group (1 row): id = String1("a")
 Row │ id       x
     │ String1  Int64
─────┼────────────────
   1 │ a            1
⋮
Last Group (1 row): id = String1("b")
 Row │ id       x
     │ String1  Int64
─────┼────────────────
   1 │ b            2

As you can see "a" and "b" are printed with String1 wrapper due to recent changes in InlineStrings.jl.

While we have:

repr("text/plain", x) is typically a "pretty-printed" version of x designed for human consumption.

which is, in general closer to what we want in this case.

After this PR the String1 wrapper is not printed.

[bkamins]

Thank you!