Include `mathvariant="normal"` for singular elements (chemistry)

[samimaattaCelia]

For proper rendering we should use mathvariant="normal" elements in chemistry. I don't think this affects MathCAT, since one of the tests uses this form.

Should be like this:

<math xmlns="http://www.w3.org/1998/Math/MathML">
    <msub>
        <mi mathvariant="normal">H</mi>
        <mn>2</mn>
    </msub>
    <mo>+</mo>
    <msub>
        <mi mathvariant="normal">O</mi>
        <mn>2</mn>
    </msub>
    <mo>&#x2192;</mo>
    <msub>
        <mi mathvariant="normal">H</mi>
        <mn>2</mn>
    </msub>
    <mi mathvariant="normal">O</mi>
</math>