Allow disabling cell optimization via command line argument (#1011)

This allows to turn off lattice parameter optimization for GFN-FF and mcGFN-FF using the --nocellopt keyword

src/geoopt_driver.f90

@@ -157,7 +157,7 @@ subroutine geometry_optimization &
      ! get number of unique species; used in precond_lindh
      !call get_identity(mol%nid, mol%id, mol%at)
      ! create new anc filter
-     optcell = mol%npbc > 0  ! optimize cell parameters if present
+     optcell = mol%npbc > 0 .and. set%optcell
      call new_cartesian_filter(filter, mol, optcell)
      ! create new Limited-memory BFGS optimizer 
      call new_lbfgs_optimizer(lbfgs_opt, env, opt_input, filter)

src/prog/main.F90

@@ -1909,6 +1909,10 @@ subroutine parseArguments(env, args, inputFile, paramFile, lgrad, &
             else
                call env%error("The wrtopo keyword is missing an argument.", source)
             end if
+
+         case ('--nocellopt')
+            set%optcell = .false.
+
          end select
          call args%nextFlag(flag)
       end do

src/setparam.f90

@@ -216,6 +216,7 @@ module xtb_setparam
    logical  :: newdisp = .true.
    logical  :: solve_scc = .true.
    logical  :: periodic = .false.
+   logical  :: optcell = .true.
 
 !  Geometry input type
    integer  :: geometry_inputfile = p_geo_coord