Route selected mathtext glyphs to Computer Modern

[cvanelteren]

Closes #717

This patch allows users to use Computer Modern font for certain latex symbols.

snippet

import ultraplot as uplt

expr = (
    r"$\mathcal{ABCXYZ}\quad"
    r"\sum_{i=0}^{n}\quad"
    r"\prod_{j=1}^{m}\quad"
    r"\int_a^b\quad"
    r"\oint_C$"
)

fig, axs = uplt.subplots(
    nrows=2,
    refwidth=9,
    refheight=1.7,
    share=False,
    span=False,
)

contexts = (
    (
        "Control: DejaVu math fontset",
        {
            "mathtext.fontset": "dejavusans",
            "mathtext.default": "it",
        },
    ),
    (
        "UltraPlot patched default: selective Computer Modern",
        {},
    ),
)

for ax, (title, rc_kw) in zip(axs, contexts):
    with uplt.rc.context(rc_kw):
        ax.text(0.02, 0.5, expr, transform="axes", fontsize=34, va="center")
    ax.format(title=title, titleloc="left")

axs.format(xlocator="null", ylocator="null", xspineloc="none", yspineloc="none")
uplt.show()

[cvanelteren]

This patch adds an opt-in mathtext.cm to the RC settings that leaves the current math rendering unchanged by default. When it is enabled it routes \mathcal through Computer Modern (cmsy10) and selected large operators (\sum, \prod, \coprod, \int, \oint) through Matplotlib's CM/Bakoma while preserving the active UltraPlot font for normal letters and numbers.

[cvanelteren]

Can you test this PR @lzhang1994 to see if this would work for you?

[codecov]

Codecov Report

❌ Patch coverage is 96.38554% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/internals/fonts.py	87.50%	3 Missing ⚠️

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