Add blockref_alloc param to default_arrmeta_construct

This allows one to choose whether blockref types should
initialize so as to create values (blockref_alloc=true),
or not (blockref_alloc=false), in which case the memory
is owned by the parent nd::array and is for viewing only.
Merge pull request #109 from mwiebe/arrmeta_defcon

Add blockref_alloc param to default_arrmeta_construct