- switching to OrdinaryDiffEq instead of DifferentialEquations in order to remove dependencies
- fixed: calling solution for a single time point and a single node returned an array with a single element instead of the actual element
- calling solve on a grid and state that do not belong to each other now raises an AssertionError
- added states from grid solution for single time points
- & fix line coverage and testing output
- adding functionality for combining multiple grid solutions to one composite grid solution