Reducing over a subset of axes

[davidhassell]

Description

Implement reductions over a subset of axes, i.e. emulating numpy's the axis keyword argument to is reduction functions (e.g. np.sum(x, axis=(0, 2)))

Closes #247

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• All tests pass

[valeriupredoi]

@davidhassell - looks great, you mind fixing the tests pls (axis is confused with storage_type since we need to correctly arg that in the test call), or I can do it? 🍺

[valeriupredoi]

OK I fixed the unit tests, could you please add tests/test_data/test1.nc since it's missing 🍺

[valeriupredoi]

many thanks @davidhassell - this is going in now 🍺