Release notes for sbmlutils 0.4.6

• simplified model factory
• improved and simplified model examples
• support of distrib with comp (flattening)
• update to libsbml>=5.19.0
• update libroadrunner>=2.0.4
• bugfix #173, deprecated xlrd replaced with openpyxl
• improved test coverage to >70%
• dropping python 3.6 support