[tutorial] more introductory words to Features
[tutorial] add untagged example code
[tutorial] add untagged example to tutorial.rst
[tutorial] link untagged feature example in tutorial list
[tutorial] change code to use the dataView features in taggedFeature.py
[tutorial] change stimulus retrieval to use retrive_feature_data
[tutorial] add plotting of retrieved roi data
[tutorial] add image for retrieved rois
[tutorial] add text for retrieve data section
[tutorial] separate design principles from tutorials
[tutorial] remove design principles from the tutorials file.
[tutorial] add overview and tutorials as separate points
adapted test cases for the changes in NIX dataAccess api
Merge pull request #99 from jgrewe/tutorial

Tutorial

LGTM