Allow vector assignments

>>>import pystencils as ps
>>>import sympy as sp
>>>a, b, c = sp.symbols("a b c")
>>>ps.Assignment(sp.Matrix([a,b,c]), sp.Matrix([1,2,3]))
(Assignment(a, 1), Assignment(b, 2), Assignment(c, 3))

Fixes #17 (closed)

pystencils/simp/assignment_collection.py

+import itertools
 from copy import copy
 from typing import Any, Dict, Iterable, Iterator, List, Optional, Sequence, Set, Union
 
@@ -43,6 +44,11 @@ class AssignmentCollection:
             subexpressions = [Assignment(k, v)
                               for k, v in subexpressions.items()]
 
+        main_assignments = list(itertools.chain.from_iterable(
+            [(a if isinstance(a, Iterable) else [a]) for a in main_assignments]))
+        subexpressions = list(itertools.chain.from_iterable(
+            [(a if isinstance(a, Iterable) else [a]) for a in subexpressions]))
+
         self.main_assignments = main_assignments
         self.subexpressions = subexpressions