Add a validation step in case of MathML input

[ghost]

This adds a validation step when input is MathML, to check if it is valid. This applies for MathML to MathML conversion as well.

[carlotm]

@fquffio I believe you have to manually enable me to run GH Actions. I think that is una tantum:

https://github.com/chialab/math-api/actions/runs/1380387383