Mea culpa: MERRA2 restarts are 03z, 09z, 15z and 21z

[mathomp4]

Back in issue #25, I mistakenly thought that we only had MERRA-2 restarts at 21z and @weiyuan-jiang duly made a change to the remapping to make sure users didn't ask for others (see 6766dd0)

Well, last week I was talking with @bena-nasa and he informed me "Nope. You are wrong!" and he was right. If you look, you'll see MERRA-2 has restarts at 03z, 09z, 15z, and 21z:

d5124_m2_jan10.fvcore_internal_rst.20231004_03z.bin
d5124_m2_jan10.fvcore_internal_rst.20231004_09z.bin
d5124_m2_jan10.fvcore_internal_rst.20231004_15z.bin
d5124_m2_jan10.fvcore_internal_rst.20231004_21z.bin

So, the right thing to do is to make users regridding from MERRA2 are specifying only these four possible hours, and either error out or something to prevent from asking for, say, 05z restarts or the like.