Release notes for sbmlutils 0.5.5

Bugfix release.

Fixes

• Fix #350, additional validation of units on object creation
• Fix #349, better error message if UnitDefinitions are not created
• Fix #351, cleanup API & removal temp files
• Fix #348, correct handling of NaN and UTF-8 encoding
• Fix #336, fixed base unit definitions
• Fix #338, improved unit rendering and definition