rework debug_print

Material.debug_print was based on a current p, T given by the last
set_state() and would return an unrolled list of minerals. This makes it
inconvenient to use and will provide the wrong information. Now it will
return a hierarchical representation independent of p,T. The old
function is now available as print_minerals_of_current_state().
update tests
Merge pull request #31 from tjhei/rework_debug_print

Rework Material.debug_print