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