Basic Vector framework

Added:

1. Vector core - Vector, VectorAdd, VectorMul, BaseVector
2. BaseScalar
3. Del
4. CoordSysRect
5. Point
6. functions.py - express, matrix_to_vector 
We could keep `orient_new()` but also make these public too. You could even maybe imagine Rotator/Orienter classes. Then you could pass in the Rotator instance to `__new__` or `orient()`.

```python
rot = BodyFixedRotator('123', q1, q2, q3)
A = CoordinateSystem('A', orient=rot)
B = A.orient_new('B', rot)
```