Description

partially resolves #2604 (following the trend of previous work in the color module for rgb2grey->rgb2gray, etc)

The first commit deals initiates a deprecation for:
greycomatrix -> graycomatrix
greycoprops -> graycoprops

My main question for that commit is the version to set for the deprecation. I have it as 1.0, but am fine with increasing it to a subsequent release if we want to give at least 2 releases before removing the old functions.

The remaining commits involve consistently using gray rather than grey in the documentation.

I can address the remaining uses of grey in functions and filenames in the morphology module in a separate PR.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.