io-test: Replaced assert() checks by if (...) throw std::runtime_error(..)
io-test: Replaced assert() checks by if (...) throw std::runtime_error(..)

assert() checks don't trigger in release mode, so not suitable for checks in tests.

viennagrid/viennagrid-dev,viennagrid/viennagrid-dev,viennagrid/viennagrid-dev