Equality operations support (and copy?)

[pp-mo]

Currently making progress with fully-feature dataset comparison, which should complete soon #27 , #70

But it is natural in code to expect equality comparison of the core objects.
In course of proposing a dim-slicing, found I wanted comparison for testing it, and copying for the operation itself. See #70

We avoided doing this because an uncritical one-purpose equality may not be very useful -- at least, on dataset and variables .
At the same time, it's confusing + unexpected that even NcDimension and NcAttribute don't behave as expected with ==.

The NcDimension and NcAttribute comparison, at least, can be quite simple + what you would expect.
Maybe the NcData / NcVariable comparison can work via compare_datasets function (probably to be renamed "dataset-difference").
Scope of all the settings (data-compare, dims-order etc) could be provided via a context control, e.g.

with ncdata.COMPARE_CONTROL.context(check_dims_order=False, check_var_data=False):
    result = var1 == var2

But obviously, this will take a bit of explaining, and might be better left as a difference function.
( Also, what should the defaults be, and do we allow direct one-time setting, e.g. COMPARE_CONTROL.check_dims_order = False
)
In any case, the best default behaviour for variable equality is not too clear. Hard to choose anything but "total equality", and user beware. But then we should also do a non-realising equality test (which we currently don't).

Also for this, we should expose variable-comparison as a separate function, which it currently is not (but again quite simple).