title: Getting mathematical function as user's input
body: <p>I need to know how transfer string input into executable function. 
  For example - user write string 'x*Sin(x**2)' and then programm takes it as function, can calculate a value for given x, can plot derivation of this function etc. I've read that there is module called scitools.stringfunction, but as far as I know this module is not callable in python-3.</p>
  
  <p>Any ideas how to make it? </p>