Turned ndim into a property of NdMapping objects
Added pop method to NdMapping objects
    Added add_dimension method to NdMappings

    The add_dimension method allows creating a new NdMapping from an old one, with a newly defined dimension.
 Added empty method to NdMappings

 The empty method creates a new NdMapping with the same parameter values and metadata as the old mapping but without any of the data.
Minor element_check fix in ndmapping

_element_check now called on every newly added item to allow checks independent of whether an enforced_type is defined
Updated ProjectionGrid methods

ProjectionGrid now performs element_check and doesn't implement its own _update_item method