Extend the check of isConstant to values that also have a grid. By this, some computations can be simplified.

Changed verification of error messages in unittests.quantity.testDiscrete/testDiffConstant2d because matlab changed error identifiers in R2020a to R2019b

misc.jordanReal: new function for transforming a dynamic matrix into Jordan form with real sub-block-matrices
+ some adaptions in methods that use this method now

it is now adopted to the implementation of the quantity.Discrete/int/cumInt

Add a specification for boundary values in quantity.Domain

quantity.Domain: new methods real and imag

quantity.Discrete/zeros will return an empty array if the requested size for zeros contains 0 as size. quantity.Discrete/compose works on quantities defined on quantity.EquidistantDomain

- misc.unique4cells: new function similar to unique, but works for arbitary cells
- misc.getMinimalInternalModel: new function to calculate minimal internal model as state space

- ode45 seems to be more precise for this kind of problems then ode15s
- successive approximation
- faster implementation for the case that the integral term can be neglected

misc.solveVolterraIntegroDifferentialIVP: new script to solve a system of volterra integro differential initial value problems

quantity.Discrete & Symbolic: bugfixes because of string inputs in plot + Symbolic.subs + Symbolic.gridName2variable

Change the parameter order of quantity.*/diff
Now, the domain name wrt which the derivative should be taken is the second argument and the order of the derivative is the third.

speed up of the unittest utilizing setupOnce