Migrate notebook links from bitbucket to github

[BenPortner]

Done:

• scanned notebooks until IO - Importing Agribalyse 1.3 with Ecoinvent 3.2 cutoff.ipynb
• migrated 2015 LCA Seminar - Class 4 - Hybrid LCA.ipynb
• migrated Defining a new Matrix - example of Weighting and Normalization.ipynb

Open problems:

• Defining a new Matrix - example of Weighting and Normalization.ipynb contains multiple links to https://bw2data.readthedocs.org/ which is not available anymore
• Getting Started with Brightway2.ipynb contains a link to Whoosh on bitbucket, which is not available anymore
• IO - Importing Agribalyse 1.3 with Ecoinvent 3.2 cutoff.ipynb contains multiple hard links to commits on bitbucket, which I cannot resolve anymore