Fix segfault

Problem was double deletion of member speed shared by two Batiment
instances. When one writes :

    Batiment batiment; // 1
    batiment = Batiment(); // 2

It creates create two batiments : 1 and 2, and copies batiment2 to
batiment1.

With Batiment { ... Bla* m ...  ~Batiment() { delete m; } ... }, this
assignment results in batiment1.m == batiment2.m.
When batiment2 is (implicitly) deleted, *batiment2.m is deleted.
When batiment1 is (implicitly) deleted, *batiment1.m is deleted... but
it already was. segfault.