WIP: implement default operators for discretization
WIP: implement default operators for discretization

- added `LinearOperator` versions of `GridCollocation` and `NearestInterpolation`
- wrote examples for `NearestInterpolation`
- fixed some minor things
- TODO: support for vectorization

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