add materials<- generic and default method
change materials.im3d method to materials.default

* since it does nothing clever
teach read.im3d.amiramesh to parse and set materials
update help/NAMESPACE for materials.default
update help for materials
Merge branch 'feature/im3d-extra-attrs'

* feature/im3d-extra-attrs:
  update help for materials
  update help/NAMESPACE for materials.default
  teach read.im3d.amiramesh to parse and set materials
  change materials.im3d method to materials.default
  add materials<- generic and default method
* closes #99