More typestable & Julian API

[j-fu]

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.