This commit introduces a new helper function
void GridProperty<>::setElement(i, val, dflt)
that handles paired assignments to 'm_data' and 'm_defaulted'[%].
This simplifies the bookkeeping and implementation of the various
assignment and value operations.
While here, also add a unit test to demonstrate expected behaviour
of the m_defaulted flag. The only really questionable setting is
what happens if the deck applies a relative operator (e.g., ADD or
MULTIPLY) to a value that would otherwise be defaulted. In the
current implementation the m_defaulted flag will remain set in this
case, but it arguably should be switched to unset (false).
[%]: Suggested by [at]akva2
a886a2334b