Explicitly state Fragment Type in FragmentType.hh enum
We should make sure that since Fragment Types should be set-in-stone for the lifetime of an experiment, that in FragmentType.hh/.cc, the enumeration values are set explicitly as offsets from FirstUserFragmentType and the names vector should be replaced with an unordered_map<type_t, std::string>.
Explicitly set the artdaq_demo Fragment Type IDs to offsets of
FirstUserFragmentType. This allows new Fragment Types to be inserted in
the list without disrupting access to previous data files (i.e. if we
wanted to add TOY3 type fragments, we can now add them after TOY2
instead of before INVALID). Update names to a map between Fragment Type