AST-1213 Use unique DPBuffer pointer and add XTensor view in ApplyBeam
Introduce a unit test for ApplyBeam::process() step and then adapt ApplyBeam and the unit test to use unique DPBuffer pointer interface and XTensor view
Introduce a unit test for ApplyBeam::process() step and then adapt ApplyBeam and the unit test to use unique DPBuffer pointer interface and XTensor view