added Class ObjectVector.

Auto vectorized functions and object generators now return an object of
class ObjectVector, which can be coerced into a data.frame with one
object per row.
Added as.data.frame methods for all parameter sets.
fixes #18.
updated timer function that allows for adding a message for a new lap.
bug fix for validating an empty ObjectVector
Added SimulationParameters class

Simulation is now handled as a method for SimulationParameters.
SimulateCKD now creates a SimulationParameters object and called
`$simulate()`.
Also added SimulationResults for returning the results.