[FEATURE] Add get and trasnform geometry in Expression

In version 2.6, new keywords has been added to Expression :
* $currentfeature - returns the current feature
* $atlasfeature - returns the atlas feature
* getFeature - gets a matching feature from a layer

But I think, Expression lacks a few keywords :
* getGeometry - returns the feature's geometry
* transformGeometry -  returns the transformed geometry

These new keywords can be used with getFeature for some geometry tests like
 intersects, crosses, contains, etc