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

except some points, that must be discussed for the quantity.Symbolic, the implementation of quantity.Domain seems to be nearly finished.

quantity.Symbolic.int: if integral bounds are variables, they are always considered in resulting quantity.

updates in misc.fold and misc.unfold concerning the preservation of the folding point or not. Unfolding should now work exactly

Haoyue

See merge request !4

Fixed #16: multiplication of symbolics is working again

misc.variables2tex : added handling of function handles that cannot be converted to symbolic. This happens when an interpolant is evaluated in the function.