Documentation updates to material
rework Material.debug_print

It now unrolls into minerals at prints a list:
Material 'Composite':
  0.95 of phase 'burnman.minerals.SLB_2011.mg_perovskite'
  0.05 of phase 'burnman.minerals.SLB_2011.periclase'
Merge pull request #9 from tjhei/documentation_updates

Documentation updates to material