Weave fails to convert markdown to pdf when env=:chemical and mathjax=true is involved

[goggle]

I found this issue while rebuilding the tutorials from DiffEqTutorials.jl.

I have a markdown test_chemical.jmd with the following content:
test_chemical.jmd

The relevant line in is this:

x = latexify(repressilator; env=:chemical, mathjax=true);

Using Weave.jl to build a PDF from it (weave(..., doctype="md2pdf")) fails:

┌ Warning: Error converting document to pdf. Try running latex manually
└ @ Weave ~/.julia/packages/Weave/UOxmI/src/pandoc.jl:121
ERROR: IOError: unlink: no such file or directory (ENOENT)

Running pdflatex on the generated .tex file, shows this error:

! Undefined control sequence.
<argument>  \require 
                     {mhchem} \ce { \varnothing &->[\frac {\alpha \cdot K^{n...
l.150 \end{align*}

The relevant section in the .tex file is this:

\begin{align*}
\require{mhchem}
\ce{ \varnothing &->[\frac{\alpha \cdot K^{n}}{K^{n} + P_3^{n}}] m_{1}}\\
\ce{ \varnothing &->[\frac{\alpha \cdot K^{n}}{K^{n} + P_1^{n}}] m_{2}}\\
\ce{ \varnothing &->[\frac{\alpha \cdot K^{n}}{K^{n} + P_2^{n}}] m_{3}}
\end{align*}

The problem is, that Weave.jl adds the command \require{mhchem} to the .tex file because of mathjax=true.

Shouldn't Weave.jl emit the line \require{mhchem} when generating the .tex file?
Or is there another way to make this example work without breaking the transformation to HTML (i.e. keeping mathjax=true when building HTML and using mathjax=false when building PDF in a static .jmd file)?

[isaacsas]

@goggle Just noticed this issue. I don't think the problem is with Weave -- the mathjax=true option is the default for latexify and I believe that is what adds the mhchem package requirement. If you set mathjax=false then if I'm remembering right the package won't get added. We need to update the tutorials to detect if Weave is making a pdf and set mathjax=false in that case.