[REF] Resolve rescale mode warnings

[mathieuboudreau]

When running tests and/or calling certain functions, the following warnings appeared:

/Users/mathieuboudreau/anaconda3/envs/ads36_venv/lib/python3.6/site-packages/skimage/transform/_warps.py:84: UserWarning: The default mode, 'constant', will be changed to 'reflect' in skimage 0.15.

This happens in two locations, in our rescaling data augmentation tools, and in raw_img_to_patches function.

I resolved in this branch this by explicitly setting the mode value to constant in the function calls.

However,

by looking at the documentation, it appears to me that setting it to reflect might be the behaviour that we actually want? @perone what do you think?

...

mode : {‘constant’, ‘edge’, ‘symmetric’, ‘reflect’, ‘wrap’}, optional
Points outside the boundaries of the input are filled according to the given mode. Modes match the behaviour of numpy.pad.

...

Notes

Modes ‘reflect’ and ‘symmetric’ are similar, but differ in whether the edge pixels are duplicated during the reflection. As an example, if an array has values [0, 1, 2] and was padded to the right by four values using symmetric, the result would be [0, 1, 2, 2, 1, 0, 0], while for reflect it would be [0, 1, 2, 1, 0, 1, 2].

Edit: mode='constant' is used explicitly in numerous other locations, so let's keep it constant here until we test otherwise. e.g. https://github.com/neuropoly/axondeepseg/blob/03e2a2c3ecd464a0a1c86710121e429489e0eaa4/AxonDeepSeg/data_management/data_augmentation.py#L174-L175

[coveralls]

Coverage decreased (-0.04%) to 80.18% when pulling 03e2a2c on resolve_warnings into 2b2a15b on master.