Feature: Simplify/resolve system of equations

[snowyu]

const expr = ce.parse(`\\begin{cases}x+y=70\\\\2x-4y=80\\end{cases}`);
console.log(expr.latex);
// \begin{cases}x+y=70&\top\\2x-4y=80&\top\end{cases}

Can a system of linear equations with multiple variables be simplified?

[arnog]

This is not currently supported, but would make a great addition.

[cgrisar]

Resolving a system of equations would be best served through matrix operations.

However, parsing has already a hard time to parse matrix operations to MathJSON (cfr. bug reporting in #251 )