New unit checking performance

[Per Östlund]

The new unit checking has rather severe performance issues. Consider a model such as this:

connector C
  Real e;
  flow Real f;
end C;

model M
  C c1[100000], c2[100000];
equation
  connect(c1, c2);
end M;

The whole instantiation, including connection handling, scalarization and DAE conversion, takes about 3 seconds for me. The unit checking meanwhile takes 22 seconds.

[Per Östlund]

Fixed by sjoelund.se in c2f24bdf, which also reactivates the flag so that the unit checking can be turned off if needed. The performance could probably be improved further, but the most severe scaling issues have been fixed at least.