More typestable & Julian API

Currently, we access data using grid[T] where T is a type. However, this is occasionaly
leads to tpype instabilities.
By defining
```
    (::Type{T})(grid::ExtendableGrid) where T<:AbstractExtendableGridApexType= grid[T]
```   
we can write both `grid[Coordinates]` and `Coordinates(grid)`. And we could specialize wrt. return types.
