Test if a draw function is a function in validate_draw_function

[twallema]

• I have checked to ensure there aren't other open Pull Requests for the same update/change
• I have updated the documentation accordingly.

Describe your fixes/additions/changes

In models/validation.py function validate_draw_function it wasn't checked if a draw function was a function before checking its number of input parameters, this opened the door to unpredictable error messages if the user would not supply a function.