Rescale allocation layer

[jankrepl]

Numerical allocation layers (e.g. NumericalMarkowitz) might generate weights that do not satisfy the constraints. There are the following reasons for it

• small floating point differences (e.g. w_i=-1e-8 with w>=0 constraint)
• algorithm did not converge and the underlying solver did not raise any exception

It can become an issue when using a loss that is not defined for certain values of w.

The cleanest solution would be to have a new allocation layer that would be called after the original allocation layer. Ideally,
it would do the following

• Perform checks and potentially raise exception
• Rescale weights to satisfy the original constraints

[jankrepl]

Related to #41