add methods Rows(), Cols(), Size(), RowView(), Contiguous(), DataView(), Data()
new file util.go with a couple new functions, as well as utility
functions moved from existing files
NewDense and MakeDense

change behavior of existing NewDense and rename it to MakeDense;
add a new NewDense;
corresponding changes to tests and other files;
new file test_util.go with utility functions for testing