version 0.3.1 release

version 0.3.1 - 20190930

• added examples for Schlieren renderer in examples/schlieren
• added examples for various data samplers in examples/sampletrace
• added second, energy-accumulation schlieren algorithm
• added 'data.state' argument to sampletrace to track data
• moved image resolution specification to Camera
• added Cinema database example of Water in the Universe data in examples/WitU
• added 'data range' to visualizations to select range of interest