use `std::abort()` instead of `assert(false)`
This addresses the following compiler warning from GCC 9:
dune/functions/functionspacebases/taylorhoodbasis.hh:179:3: warning:
control reaches end of non-void function [-Wreturn-type]
This addresses the following compiler warning from GCC 9:
dune/functions/functionspacebases/taylorhoodbasis.hh:179:3: warning:
control reaches end of non-void function [-Wreturn-type]