58 nf speedup cli

[forman]

This partly addresses #58, as cate --version and cate -h now perform much faster.

Performance improvement comes from two optimisations:

1. cate.util does not aggregate sub-packages anymore. We have to explicitly import now. This is because some sub-packages imports are slow, notably the new cate.util.opimpl.
2. CLI commands in cate.cli.main now only import required modules when they are executed.

[codecov-io]

Codecov Report

Merging #467 into master will decrease coverage by 0.01%.
The diff coverage is 81.25%.

@@            Coverage Diff            @@
##           master    #467      +/-   ##
=========================================
- Coverage   76.92%   76.9%   -0.02%     
=========================================
  Files          74      74              
  Lines       10438   10497      +59     
=========================================
+ Hits         8029    8073      +44     
- Misses       2409    2424      +15

Impacted Files	Coverage Δ
cate/util/cache.py	81.78% <100%> (+0.07%)	⬆️
cate/webapi/websocket.py	31.39% <100%> (+0.4%)	⬆️
cate/ops/outliers.py	100% <100%> (ø)	⬆️
cate/ops/correlation.py	98.91% <100%> (ø)	⬆️
cate/core/op.py	91.98% <100%> (+0.12%)	⬆️
cate/core/workflow.py	93.58% <100%> (+0.02%)	⬆️
cate/core/workspace.py	87.12% <100%> (+0.13%)	⬆️
cate/webapi/wsmanag.py	28.49% <100%> (+0.4%)	⬆️
cate/core/wsmanag.py	65.32% <100%> (+0.25%)	⬆️
cate/util/process.py	90.15% <100%> (ø)	⬆️
... and 16 more

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[JanisGailis]

This is a general comment. Is there a reason why we use relative imports in some places?

As for this PR - looks good.

[mzuehlke]

Speedup is always a good thing.
All the new flake8 warnings are from flake8 3.5.0. a new version with new checks.
We can fix these later.

[forman]

@JanisGailis we prefer relative imports over absolute ones because they do not force loading the parent package's imports from ./__init__.py.