New rule: math-mtable-one-cell

[TimArborealis]

Suggested name: math-mtable-one-cell

Sometimes math content is marked up in an <mtable> when it should not be. To check for this, we need a rule which catches instances of <mtable> with one single cell.

The validator should catch all instances of <mtable> which contain exactly one <mtd> element.

Example:

<math xmlns="http://www.w3.org/1998/Math/MathML">
    <mtable>
        <mtr>
            <mtd>
                <mn>2</mn>
                <mo>+</mo>
                <mn>2</mo>
            </mtd>
        <mtr>
    </mtable>
</math>

See more examples in test_issue_21.epub

@MartheGjelstad, thoughts? (created this issue with @AntonLMTM )