datatype: correctly support MPI_Pack_external when heterogeneous supp…

[ggouaillardet]

…ort is enabled

[ggouaillardet]

@bosilca could you please review this ?
I used a logic similar to unpack, so MPI_Pack_external can do the byte swapping if needed.

currently, this works only with --enable-heterogeneous.

In order to get this working without --enable-heterogeneous, I will have to always compile some code that is currently used only with heterogeneous cluster.
this will be a bit more of code which is always executed, but that is only needed for MPI_{Pack,Unpack}_external.
an other option is to duplicate some code without --enable-heterogeneous.

can you please advise ?

[ggouaillardet]

@bosilca
i pushed a second commit to fix openmpi without --enable-heterogeneous
bottom line, i just had to make OPAL_CONVERTOR_COMPUTE_REMOTE_SIZE the same regardless heterogeneous support, which is a much smaller change than i anticipated.

[bosilca]

@ggouaillardet I made a different patch that enables the heterogeneous mode for the OPAL datatype in all cases, and adds support for more complicated datatypes. Take a look at #1394 .

[ggouaillardet]

this topic is now handled is #1394, so i am closing this PR