remove gsl; true c++11 compatiable

1.3.8

remove gsl, really

remove std::optional dependence; downgrade to c++11

1.3.7

replace optional<> with pointer

Remove xtensor dependency

1.3.6

remove xtensor dependency

remove xtensor from interface

1.3.5

use new EllAlgo 1.3.6

update according to new Ell, add set_witness_vec()

1.3.4

update Ellalgo version to 1.3.5; add set_witness_vec()

use assess_optim() and assess_feas() instead of opeartor()()

1.3.2

fix style

Rename some objects and functions

1.3.1

rename some objects and functions

Cut is now a pair rather than a tuple

1.3

fix regressions

fix style and documentation

1.2.1

fix style and documentation

Updated third-party libraries

1.2

set version 1.2