changed "minimum" to "infimum" in betweenness

reflects the fact that we actually compute the infimum rather than the
minimum across all alpha-cuts; for backwards-compatibility, also
"minimum" is still accepted

reimplementation of _intersection_mu_special_case in Concept class

- old code could not take into account intersection of raw cuboids with
different values of mu
- needed to re-implement _intersection_mu_special_case in order to deal
with this
- some changes to _intersect_fuzzy_cuboids (only use crisp intersection
if mu identical, different extrusion, bounding box for result)
- additional test cases, some adapted test cases (due to new corrected
behavior)

small documentation update

updated documentation, improved robustness of integral-betweenness

Update README.md

Updated 2D fruit space image (corrected alpha cuts)