Fix memory allocation on MPI gather_abundance()

[richardreeve]

Trying to fix some MPI problems in #66 has led to an issue with gather_abundance() doing a lot of allocation while trying to collect the abundance data sensibly onto one node. It seems to be non-trivial to fix, but doesn't get done very often, so this issue just notes it needs fixed when possible. This might involve one or many of:

1. Some kind of custom view into the full abundance array on the root process where the abundances are gathered to
2. Some kind of custom view into the process-specific abundance array on every process
3. Distributed calculation instead of whatever is being done on the root process

I suspect 1 may be the easiest to accomplish, then the array can be copied as is from the other processes, but 3 might be the ideal solution in many cases.