improve testing and test coverage

[orbeckst]

Add more tests to cover all core modules

• contacts
• gibbs (minimal tests added in PR support combining contact timeseries from multiple repeats #16 )
• kinetics
• util

Other modules

• wdensity (would this be covered with kinetics?)
• pwdensity (are both modules needed?}
• combine (this is really a script and should be removed with add CLI script basicrta #20 )