Deprecate various functionality in GROMACS 2019

Deprecate various functionality in GROMACS 2019

Published a deprecation policy.

Updated the release notes to refer also to previously deprecated
features.

Announced intent to change some functionality:
* gmx mdrun -membed options (but not feature)
* gmx mdrun -rerun option (but not feature)
* integrator .mdp field will contain only integrators
* gmx do_dssp to be replaced by gmx dssp
* gmx trjconv and friends to be split and rewritten

List of newly deprecated functionality:
* conversion of aromatic rings to virtual sites
* gmx mdrun -table options (but not feature)
* gmx mdrun -gcom option and feature
* gmx mdrun -nsteps option and feature
* gmx mdrun -nsteps -resetstep -resethway moved to
  a gmx benchmark tool
* gmx mdrun -confout removed

Also updated release notes for functionality removed in GROMACS 2019.

Refs #2495, #1781
Fixes #2569, #1925

Change-Id: I1d00859d0f15409a472984f5a65347a50c71ad17