Make set_dimension() and set_nnodal_position_type() public

This is required to be able to set element geometry properties in a factory
function rather than a constructor.

These are only access functions, which are ususally public, so this change
is consistent with other similar functions.
Add functions to construct SumOfMatrices lookups from an array of ints

* This is the data format for HLib's lookup schemes, so this is useful for
  creating hierarchical matrices with some non-hierarchical
  additions. E.g. for BEM corner contibutions.