Finished basic `Discretization` class
Finished basic `Discretization` class

- wrapped `ntuples` instead of subclassing it
- wrote pass-through attributes `dim`, `dtype` and `field` (in `LinearDiscretization`)
- fixed `equals()` and `contains()` (untested though)

aringh/odl,kohr-h/odl,kohr-h/odl,aringh/odl,odlgroup/odl,odlgroup/odl