[container] fix handling of num_mutexes == 0

Shifted Qr-Solver improvements

Improve vector lock guard, other small changes

[common.matrix.sparse] do not divide by zero

[tests] add BlockedFieldMatrix to tests

Fix a missing return, bump .vcsetup

This is not actually necessary. Adding a handler for std::exception also
hid the more specialized handlers provided by pybind11 itself.