Improve connectivity construction's performances

Step 1: get rid of a bunch of map-like structures

doc/userdoc.org

@@ -1223,8 +1223,7 @@ are sorted by type of left hand side variable.
 Observe that for these small matrix types ($\mathbb{R}^{d\times d}$) the
 construction ~A *= B;~ where ~B~ is a matrix of the same type as ~A~ is not
 allowed. The main reason for that is that for $d>1$ this operation has
-no interest since it requires a temporary. One will see bellow that it
-is possible to write ~A = A*B;~ if needed.
+no interest since it requires a temporary. One will see below that it
 #+END_note
 
 - ~string~: the ~*=~ operator is not defined for left hand side string variables.