do not specialize on the function in adapted_grid

[KristofferC]

This avoids having to recompile it for all different input functions

Before:

julia> @time adapted_grid(x->x, (0,1));
  0.748009 seconds (1.60 M allocations: 82.982 MiB, 3.17% gc time, 99.26% compilation time)

julia> @time adapted_grid(x->x, (0,1));
  0.490260 seconds (593.23 k allocations: 30.643 MiB, 16.33% gc time, 99.97% compilation time)

julia> @time adapted_grid(x->x, (0,1));
  0.488363 seconds (593.23 k allocations: 30.639 MiB, 9.39% gc time, 99.97% compilation time)

After:

julia> @time adapted_grid(x->x, (0,1));
  0.732140 seconds (1.04 M allocations: 54.736 MiB, 3.45% gc time, 98.40% compilation time)

julia> @time adapted_grid(x->x, (0,1));
  0.044722 seconds (113.41 k allocations: 6.245 MiB, 99.04% compilation time)

julia> @time adapted_grid(x->x, (0,1));
  0.044496 seconds (113.41 k allocations: 6.246 MiB, 99.29% compilation time)

[KristofferC]

cc @t-bltg

[t-bltg]

Sorry I missed that PR, thanks !
Will wait for #118 to be fixed before bumping a new version.