Make types uniform in the Python exposition

Some problems/algorithms seem to have an inconsistent python exposition in terms of types.

For instance, in `../src/problems/dtlz.cpp` we have:
```
dtlz::dtlz(unsigned prob_id, vector_double::size_type dim, vector_double::size_type fdim, unsigned alpha)
```
But in the `../pygmo/expose_problems_0.cpp` it is exposed as:
```
dtlz_p.def(bp::init
```
Instead of:
```
dtlz_p.def(bp::init ...
```
A check for all these type consistencies seems thus necessary.