Codeformatting with black

[MuellerSeb]

To be in line with the GeoStat-Framework standards, I reformatted the code base with black by executing:

black pykrige/

Also applied to docs, tests and examples.

Included minor updates:

• include GSTools in tests
• adopt GridSearchCV to be future prove
• add black check stage in travis
• update array comparison in tests to be future prove

[rth]

Great! Might be worth adding a check to enforce this in CI,

black --check pykrige/ examples/ tests/

[MuellerSeb]

I updated travis and testing accordingly. When CI succeeds, I'll merge this.