Small fixes

See merge request !57

Printing backtrace directly from the program is not very useful, because
it does not include line numbers, so we still had to use the debugger.
Furthermore, the backtrace tends to be very long, especially when
multiple MPI processes crash, which obfuscates the real error messages.

Fixed SegmentView types in CSR and ChunkedEllpack.

See merge request !56

Ik/matrices csr

See merge request !55