validation of input dimensions sizes

[rabernat]

It is currently possible to create Grid and Axis objects whose sizes are not compatible with xgcm's data model.

This should not be allowed

>>> ds = xr.Dataset(coords={'x_c': (['x_c',], np.arange(1,10)),
                            'x_g': (['x_g',], np.arange(0.5,9))})
>>> ds
<xarray.Dataset>
Dimensions:  (x_c: 9, x_g: 9)
Coordinates:
  * x_c      (x_c) int64 1 2 3 4 5 6 7 8 9
  * x_g      (x_g) float64 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5
Data variables:
    *empty*
>>> grid = Grid(ds, coords={'X': {'center': 'x_c', 'outer': 'x_g'}})
>>> grid
<xgcm.Grid>
X Axis (periodic):
  * center   x_c (9) --> outer
  * outer    x_g (9) --> center

According to our own docs, the outer position should have a length of 10. So this (and all similarly erroneous Grid / Axis creation operations) should raise errors.

[jbusecke]

I think this is relevant to #208. We should fix both with one PR.

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[jbusecke]

This definitely needs fixing.