AST-1466: [DP3] Remove [nodiscard] from constructors

Removed nondiscard attribute from constructors. Additionally, removed also from one method whose return value could be safely ignored, because is within the spirit of this ticket. Before these modifications the compilation generated 1099 warnings. After these modifications the compilation generated only this warning:

dp3/DP3/common/BaselineSelect.cc:139:33: warning: 'casacore::Table casacore::TableExprNode::table() const' is deprecated: Use getTableInfo().table() instead [-Wdeprecated-declarations]

It is not fixed because it is out of the scope of this tickect.