title: Make all symbols commutative in a sympy expression
body: <p>Say you have a number of non commutative symbols within a sympy expression, something like</p>
  
  <pre><code>a, c = sympy.symbols('a c', commutative=False)
  b = sympy.Symbol('b')
  expr = a * c + b * c
  </code></pre>
  
  <p>What is the preferred way to make all symbols in the expression commutative, so that, for example, <code>sympy.simplify(allcommutative(expr)) = c * (a + b)</code>?</p>
  
  <p>In <a href="https://stackoverflow.com/a/47760281/4063051">this answer</a> it is stated that there is no way to change the commutativity of a symbol after creation without replacing a symbol, but maybe there is an easy way to change in blocks all symbols of an expression like this?</p>