[MITgcm-support] bug in obcs-sponge (added & fixed)

[Jean-Michel Campin]

Hi,

A bug in with the OBCS-sponge code (pkg/obcs) was introduced 
on Oct 25, 2010 (in dynamics.F CVS version 1.151, part of checkpoint62n), 
when I commented out the call to S/R OBCS_PRESCRIBE_EXCHANGES. 

It appears that using obcs-sponge requires an exch of gU,gV
which was done in obcs_prescribe_exchanges.F, but missing after 
Oct 25 modification. The obcs-sponge code is not tested regularly, 
so I did not realize that there was a problem.

I fixed this problem yesterday (Nov 17) by adding back the call to
OBCS_PRESCRIBE_EXCHANGES in dynamics.F.

So, if someone downloaded or updated the code between Oct 25 and Nov 17
(or downloaded checkpoint62n), OBCS-sponge will not work, and a new 
update is needed to fix it. Sorry for that. 

Cheers,
Jean-Michel