Improve package dependency management

[hans-permana]

This is based on the guideline written by @forman in CCI-Tools project (CCI-Tools/cate#139)

Dependencies should be expressed such that PACKAGE>=MAJOR.MINOR[.PATCH]<MAJOR.(MINOR+1) achieving API compatibility according to SemVer. PATCH should ideally be omiited if not required for some reason.

In only rare cases we should rely on a specific version such that PACKAGE=MAJOR.MINOR.PATCH. For example, this could be the consequence of fixing a dependency conflict. In any of such cases, the reason for doing so must be documented (e.g. comment line).

Any transitive dependencies should not be specified at all unless there is special need for this. A dependency is non-transitive if we import it explicitly in Python code or we need it for some special behaviour (e.g. dask used by xarray) or it provides a special, dynamically loaded plugin.

These are the places where dependencies are declared. All configurations should share the same dependency specifications:

• setupy.py
• .travis.yml
• environment.yml in dedop-core, currently used by Sphinx only
• Conda recipes in dedop-conda
• Constructor installer configuration