Feature/math sqrt

1) Replace calls of ::sqrt by boost::geometry::math::sqrt
2) Fix an unused typedef warning missed by Adam
3) Remove obsolete include in offset algorithm (extensions)
@awulkiew This code is relevant for non-fundamental number types. So, yes I expect an ambiguity when you have a non-fundamental number type T, for which sqrt(T) is defined in more than one namespace among the three I list in the comments. I doubt it, though, that the sqrt implementation for a number type would be implemented in more than one namespace (same for other mathematical functions).