Add support for TileMatrixSetLimits.

- Remove the `tilematrixsets` attribute from layers.
- Adds a new `tilematrixsetlinks` attribute to layers. It's a dictionary mapping matrix set names to `TileMatrixSetLink` instances.

Examples of the new classes:
```
>>> print tilematrixsetlink
<TileMatrixSetLink: EPSG:900913, tilematrixlimits={...}>
>>> print tilematrixsetlink.tilematrixlimits['EPSG:900913:6']
<TileMatrixLimits: EPSG:900913:6, minRow=18, maxRow=22, minCol=29, maxCol=32>
```