Address Clang 8 warnings. (#867)

* Remove or implement move ctor/assign operations for ostream-derived classes.
* Add explicit test for a zero row in modcc's sparse and linear solver finalize routines.
* Forward declare `morphology_impl` as a struct.
* Remove redundant capture in a lambda expression in fvm_layout.cpp.
* Add extra parens in an assign-and-test condition in mechcat.cpp.