Buffers: explicitly construct w/ and w/o mem
Buffers: explicitly construct w/ and w/o mem

We define explicitly two constructors:
1. Buffers(int n, size_t size): full buffer allocation
2. Buffers(int n, size_t size, const std::vector<T*>& mem): construct
based on a given vector of buffers => no allocation.

The `get_mem()` returns const reference to the vector of buffers instead
of its pointer.

vrancurel/ntl,vrancurel/ntl,vrancurel/ntl