Fix warning when setting boundary ID

[agzimmerman]

I'll need help from the deal.II mailing list on this one. There seems to be an issue with the data type used for boundary_id. It doesn't play nicely with ParameterHandler.

/peclet/source/pde_model.h:531:72: warning: narrowing conversion of ‘boundary’ from ‘unsigned int’ to ‘unsigned char’ inside { } [-Wnarrowing]
std::settypes::boundary_id dealii_boundary_id = {boundary};