dev: fix/add some test context methods

* also move move all im3d core tests together
Teach im3d to accept additional attributes via ...

* and include a test
Fix docs for (as.)im3d
add basic implementation of materials functions
Merge branch 'feature/im3d-extra-attrs'

* feature/im3d-extra-attrs:
  add basic implementation of materials functions
  Fix docs for (as.)im3d
  Teach im3d to accept additional attributes via ...
  dev: fix/add some test context methods
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
Add quick start section

includes sample data from Grosjean et al 2011
Add some images linked to papers that used nat

* or more exactly AnalysisSuite
link to frulhns package
add link/image for Grosjean 2011
fix nat.examples url
fix failure to export graph constructor
Fix and test ability to set NeuronName when no InputFile is specified

* fixes #100