uniform algorithm API

[mdrocco]

For uniformity with the current status of code (see discussion for PR #119), I think we should remove any policy-less function interface from algorithm.h and numeric.h.

In the current version, calling shad::min_element with shad::distributed_sequential policy does not compile, I think this is confusing for the user.

@VitoCastellana @mminutoli

[mminutoli]

We need to schedule a design meeting.