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

quantity.Discrete: rearranged on() such that the easy cases are faster, since the permutation of the grid / domain can be skipped

Further minor fixes ...

Resolve "quantity.Domain"

Closes #14

See merge request !5

Thus, also the TimeDelayOperator is working for such functions.

It uses the isequal comparison now to check if the objects have the same entries and not the == as before.
Changed type of quantity.Domain to handle

Fixed quantity.*/diff to be callable with a quantity.Domain. Secondly,  the quantity.Discrete/diff and quantity.Symbolic/diff were merged so that the common part is in quantity.Discrete/diff and the particular part for each class is in quantity.*/diff_inner