`Assignment Collection for y[0,0,0]` is usually not very helpful.

New representation:

```
In [6]:     forward_assignments = pystencils.AssignmentCollection({
   ...:         z[0, 0]: x[0, 0] * sympy.log(x[0, 0] * y[0, 0]),
   ...:     y[0,0] : x[1,1] +1
   ...:             })

In [7]: forward_assignments
Out[7]: AssignmentCollection: z_C, y_C <- f(x_C, x_NE)
```

AssignmentCollection can be used in many cases where you could also use
a List[Assignment]. With AssignmentCollection.__bool__, an empty
AssignmentCollection is falsy and a non-empty one truthy.

So you can `assert assignments, 'must not be emtpy'`

- how could this not have been caught by the tests?!

- allow AST nodes in assignment collection to e.g. put RNG nodes in
  LB method

When introducing new people to pystencils it's often simpler not to
differentiate between `main_assignments` and `subexpressions` in the
beginning.
Also for simple kernels subexpressions are often not needed, since
intermediate symbols can also be set in main_assignments.

Subexpression should be kept for expert users.

This restructuring allows for easier separation of modules into
separate repositories later. Also, now pip install with repo url can be
used.

The setup.py files have also been updated to correctly reference each
other. Module versions are not extracted from git state

- assignment collection
- sympyextensions

- Previously sympy.Eq was used to represent assignments. However Eq
  represents equality not assignment. This means that sometimes sympy
  "simplified" an equation like a = a  to True,
-> replaced sp.Eq by pystencils.Assignment everywhere
- renamed EquationCollection to AssignmentCollection

- shortened the longest running tests, moved some configurations
  to longrun

- generates a lattice model and a sweep
- support for refinement parameter scaling

Michael Kuron authored Feb 13, 2017 and Martin Bauer committed Feb 13, 2017

This commit makes the Python code backwards compatible down to Python 2.7. Previously it would only run on Python 3.5 and up.

Problems fixed included:
- `time.perf_counter()` doesn't exist
- all classes need to be new-style
- `functools.lru_cache` doesn't exist
- only the last argument to a function call can be `*`-expanded
- the `nonlocal` keyword doesn't exist
- metaclasses are used with a different syntax
- `yield from` doesn't exist
- `tempdir.TemporaryDirectory` doesn't exist
- iterators need a `next()` method