MathML reader:  unknown symbols default to ESymbol Op, not MathOperator.

This gives better behavior, e.g. an idempotent mathml -> mathml
translation of <mo>(SYMBOL)</mo>.
TeXMath writer: always use \operatorname for unknown operators.
Updated tests
Test suite - better label on diff
TeXMath writer:  Render operators disguised as identifiers.

If an EIdentifier isn't a single token, treat it as an EMathOperator.
This gives the best results for conversion from MathML, which doesn't
have anything directly corresponding to EMathOperator and represents
things like "sin" and "cos" in "mi" tags.

Updated tests.