Retrieve the function specified by the given object.
f Any object.
func The function object if one is represnted by the input, and null otherwise.

If the input is a string, an attempt is made to load this function from the files in the current path. If a function with this name is found, it is the output of the function, otherwise an error condition is created.

If the input is a function object, the input is returned.

In all other cases, and error condition is created that the input is not a function.
>>// Load 'zeros' from m-file if necessary.
Builtin Function:cos
Error The variable Anyfunction is not a function
Error in call to getFunction