On unary operators

[rll2021]

Kai and I have had an off-line discussion along the following lines. I want to publish my thinking before submitting code changes.

The main branch insists on binarizing unary operators. We do this by prefixing cos(x), say, by +,-,*,/. If we want balance among the four arithmetic primitives, then all four variations must appear in the operators_XXX.csv file.

This seems to me unnecessary. Two changes are required in fx_eval_label():
1. Treat unary operators as left prefixes. Emit op+'('+arg+')'. Walk this node pre-order.
2. Parenthesize the arguments of (infix) binary operators. Emit '('+arg1+')+op+'('+arg2+')'. Walk this node in-order.

There are knock-on effects elsewhere in the code. These arise from the assumption that all operators are binary. I have found most (all?) of them. Fixing them permits unary minus also to work as expected.

The ternary case (if/then/else) eludes me. I know of no equivalent syntax that sympy.sympify will accept. I propose replacing this code with an easily interpretable error message.