Start() doesn't return correct values when having two datasets and two vars
In ex1_8, the first Start() call has two datasets (Amon, Omon) and two variables (tas, tos). Amon has both tas and tos while Omon only has tos. In theory, the output should have values except at [dataset = 2, var = 1]. But it doesn't have data at [dataset = 2, var = 2] as well.
Besides, the printed outputs in ex1_8 are wrong. Need to fix them after the bug is fixed. Parameter
metadata_dims should be used to get both tas and tos in metadata. The corresponding unit test (test-Start-two_dats.R) is also needed to be fixed then.