closes #49

quantity.sort: renamed old sort into sortDomain and implemented new sort method that actually sorts the quantity-array according to its values.

bugfix 4 isConstant of quantity.Symbolic: Previously, isConstant behaved differently for Discrete and Symbolic. Furthermore, this caused errors, in on() of Symbolic.

commit c4687c8d45d9d9f53c8384a1194a833ccd258cc1
Author: Ferdinand Fischer <ferdinand.fischer@uni-ulm.de>
Date:   Tue Sep 1 13:55:27 2020 +0200

    fixed a bug in the evaluation of quantity.Symbolic on numeric grids,

commit dc784651
Author: Ferdinand Fischer <ferdinand.fischer@uni-ulm.de>
Date:   Mon Aug 31 13:55:01 2020 +0200

    fixed a bug in the evaluation of quantity.Symbolic on numeric grids,

quantity.Domain/find returns now "empty" values if the domain is not found. AND Cast of a quantity.Domain into a quantity.Function object.

quantity.Symbolic: new methods ones & eye

+ wrote unittests (also for quantity.Symbolic)

Fixed bug in quantity.Symbolic/subs which ourred if (z, zeta) domain should be exchanged with (zeta, z).

- unittests for quantity.Domain/Discrete and /Symbolic
- deleted unittests.quantity.testSymbolic/testSolveOde

-> unittest for testDiscrete are working

-> unittest for testDiscrete are working

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

quantity.Domain: new methods real and imag

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

changed misc.diag_nd to use a for loop on the domain, which will be collapsed.

Added misc.rat to compute rational approximations of real numbers

Added quantity.EquidistantDomain to simplify the initialization of a domain with a certain step size

Fixed signals/SmoothStep to use spline wise interpolation instead of symbolic computation