Bounds Checking, humidity and temperature calculations

This Merge Request:

• adds a FunctionCollection to group similar functions with the same output quantity together, closes #15 (closed)
• adds functions to calculate several humidity measures (mixing ratio, specific humidity, absolute humidity, water vapour pressure, relative humidity), closes #16 (closed)
• adds functions to calculate virtual temperature
• adds a bounds.ensure-decorator similar to units.ensure to add sanity checks for input and output values of functions, closes #17 (closed)
• improves the documentation with auto-generated pretty tables for units.ensure and bounds.ensure
• Updates the contribution guidelines