Mark OLESEN authored Sep 27, 2023 and Andrew Heather committed Oct 11, 2023

- earlier deletion of unpopulated HashTable on resizing:

  If the table is already clear (ie, has no entries),
  can immediately remove the old internal table before reallocating
  the newly sized table, which may avoid a needless memory spike.

- reserve() method:

  Naming and general behaviour as per std::unordered_map.
  It behaves similarly to the resize() method but is supplied the
  number of elements instead of the capacity, which can be a more
  natural way of specifying the storage requirements.
  Additionally, reserve() will only increase the table capacity for
  behaviour similar to DynamicList and std::vector, std::string etc.

  Old:
     labelHashSet set;
     set.resize(2*nElems);

  Now:
     labelHashSet set;
     set.reserve(nElems);

- remove unused HashTable(Istream&, label) constructor

STYLE: static_cast of (nullptr) and std::fill_n for filling table