Feature Request: Include all possible functions in is_function()

[lfrommelt]

I sampled functions based on the complete function space from tree.FUNCTIONS, so for instance sinh could be drawn. If I want to re-initialize a function from sympy, the dafault function check checks only the default functions. So I did the following:

function_space = list(equation_tree.tree.FUNCTIONS.keys())

equation=EquationTree.from_sympy(sympy_equation, function_test=lambda f: f in function_space, variable_test=lambda x: "x" in x)

So everything works as intended, it would however be more convenient and straight forward, if the default function check defaults.is_function() would be based on the complete function space as well and not on the defaults.DEFAULT_FUNCTION_SPACE.

I think in principle the from_sympy()-function should raise an error, if parts of the input cannot be parsed. (In my case "sinh" was silently interpreted as "sin".)

Cheers,
Leonard