[fwd.hpp] add deprecations for uses of Eigen::aligned_allocator

[ManifoldFR]

aligator now requires C++17, hence there is no longer any use for the Eigen::aligned_allocator<T> template class. Hence all aliases for the relevant std::vector type and use of std::allocate_shared (which takes an allocator type as argument) are useless.

This PR deprecates both the typedef for std::vector<T, Eigen::aligned_allocator<T>> and the aligator::allocate_shared_eigen_aligned function template, and removes all use of the later (replaced by vanilla make_shared).