Saving qsymm models

We add instructions on how to save and load a Qsymm.Model by converting the sympy representation to a string. This is a direct solution, instead of using JSON serialization.

Closes #34 (closed)

environment-minimal.yml

@@ -5,7 +5,7 @@ dependencies:
     - python=3.7
     - numpy=1.16.2
     - scipy=1.1.0
-    - sympy=1.4
+    - sympy=1.3
     - tinyarray=1.2.3
     ## Dev Dependencies
     # Testing