This commit introduces a new helper template class,
FlatTableWithCopy<RecordType>
which wraps a vector<RecordType>, provides constructors from
DeckKeyword and initializer_list<RecordType> and handles copying
tables for all-defaulted records.
We reimplement the PVTW, DENSITY, and GRAVITY tables in terms of
this helper class to reduce duplication. If the copy behaviour is
generally useful/needed we expect to replace the existing FlatTable
mechanism with the new helper.
Suggested by: Markus Blatt
254b2e2862