order to replace vcat by push!.

The benchmarks are at least equal, and better when the buffer size is
small. However, no significative difference of performance with adequate
buffer size.

My opinion the gain can be seen when simulations are going to be very
long with more complicated models than we have implemented for now.

All tests passed.

Major fix of type unstabilities which improved performances.
For now on, we observe for pygmalion benchmarks:
+2x speed simulation compared to simulation of models.
100x speed in terms of reading values (no allocations).

Implementation of the lp distance for trajectories.
Add of unit tests for lp distance.

- Add of benchmark scripts that compares perf wrt pygmalion.
=> As expected it is bad in terms of reading cost (because of row by row
matrix read) but it is also not good in terms of simulation cost.
hcat seems to perform badly, should investigate.