Fix problem or raise error when storing wavefields across processor boundaries

[krischer]

This relates to issue #29. If this is not fixed until the next release, can you please raise an error if someone attempts to calculate a database where this is problematic? I think this can done at the very beginning of the meshing stage after parsing the input files.

As (I hope at least) the usage of instaseis will increase more and more people will want to calculate their own databases and we really don't want them to calculate problematic ones.