Enable GMXAPI for GROMACS 2019.

https://gerrit.gromacs.org/c/8801 disables GMXAPI by default. We
presume this will be reverted before final release, pending resolution
of the issues referenced in the Gerrit change, but we need to update
our docs and CI testing.

ci_scripts/install_gromacs_2019.sh

@@ -30,6 +30,7 @@ pushd $HOME
          -DGMX_DOUBLE=$GMX_DOUBLE \
          -DGMX_MPI=$GMX_MPI \
          -DGMX_THREAD_MPI=$GMX_THREAD_MPI \
+         -DGMXAPI=ON \
          -DCMAKE_INSTALL_PREFIX=$HOME/install/gromacs_2019 \
          ..
    make -j2 install