[tutorial] changed example code to use the retrieve_feature_data method
[tutorial] add section for retrieving feature data
[tutorial] add something about sources.
[tutorial] add sources to toc
[tutorial] simplified section headings
[tutorial] add example for source and metadata
[tutorial] add example code for metadata handling
[tutorial] change to add metadata directly to the data array
[tutorial] add section about metadata
[tutorial] fixed code-block error
[tutorial] add example image
[tutorial] add image to tutorial, add metadata table to example code
Merge pull request #112 from jgrewe/tutorial

Tutorial

Impressive!
Merge remote-tracking branch 'upstream/master' into python3