apply nomalization to rgb data along the time dim

[zxdawn]

See this discussion: #76

[aazuspan]

Thanks @zxdawn! The failed check is unrelated to your change, but I ran tests locally and test_normalize is failing. That test (below) uses a DataArray without a variable dimension, which should be supported.

wxee/test/test_xarray.py

Lines 60 to 68 in 1c41593

	def test_normalize():
	"""Test that a normalized DataArray is in the correct range"""
	ds = xr.load_dataset(os.path.join("test", "test_data", "COPERNICUS_S2_SR_test.nc"))
	da = ds.B4
	da_norm = da.wx.normalize(stretch=0.9)
	assert da_norm.max().values == 1.0
	assert da_norm.min().values == 0.0

Instead of specifying which dims to use, it will probably be safer to just use everything but time. Something like this should work:

reduce_dims = set(da.dims) - {"time"}
min_val = da.quantile(1 - stretch, dim=reduce_dims)