Add a WMTS tilegrid
Add a WMTS source
Add example for WMTS source
Export methods methods for ol.Extent

so that wmts example work (it makes use of extent.getTopLeft() method)
Ensure supportedCRS property won't be renamed

this property was renamed when using wmts parser when built with
ADVANCED_OPTIMIZATIONS
Add a ol.source.WMTS.createFromCapabilities function
Add wmts-from-capabilities example

it demonstrates how to create a wmts source by retrieving its configuration
from a wmts getCapabilities.
Add missing exports

so that wmts-from-capabilities example works.