Equality for Locset and Region

[thorstenhater]

So far, to decide whether two region were equal, we had to rely on str(A) == str(B).
This is slow, inconvenient, and somewhat misleading.
Thus, this PR introduces equality on those types under the following definition:
Two regions/locsets are equally iff they share the same construction.

Example:

(tag 1) == (tag 1)
(tag 1) /= (region "soma") ;; even if "soma" is defined as (tag 1)

This is somewhat stricter than one might expect, but for a complete
definition, one would need to convert these expressions to their concrete
representations using the morphology and labels, which is not desirable
due to performance reasons. And could also end in surprise when
two unrelated expressions are equal given a very specific morphology.