ParamExpressionTimeAndNode: support data requirements #250
Description
Currently, params made from sympy expressions do not support arbitrary data attributes; you can only use symbols relating to simulation properties: day, node, total days, total nodes. This would be a nice-to-have feature when subclassing ParamExpressionTimeAndNode.
The user would need a way to acquire the symbols to refer to its requirements, and evaluate() would need to include attribute symbols in lambdify(), and values in fn().