Migrate configuration files to YAML

[dzhoshkun]

This is a sub-task of #166

For a roadmap, see the following comment: #166 (comment)

Basically:

• deprecate the current INI parser
• implement a new parser along the lines of:
  • loading, and
  • parsing functionalities.

[dzhoshkun]

• modify the CI script to read (only?) YAML files

[dzhoshkun]

Deprecation warnings are ignored by default, hence no user output in the above build. Apparently this decision has been (and continues to be) strongly debated.

[dzhoshkun]

• use UserWarning instead?

[dzhoshkun]

• compare new YAML files on this branch to INI files on dev branch before merging! (by reverting 9666eb9)

[dzhoshkun]

• remove INI files?

[dzhoshkun]

• default CI sections do not load any configuration files, should test these changes using the slow tests etc.

[dzhoshkun]

• the last commit fails because there are hard-coded filenames featuring the .ini extension. Those should be replaced, possibly using a global file extension constant

[dzhoshkun]

• references to / examples of INI files in documentation should be revised as well

[dzhoshkun]

• depends on Migrate configuration files to YAML NiftyNetModelZoo#2

[dzhoshkun]

• set the environment variable niftynet_model_zoo to point the dowload utility to the Migrate configuration files to YAML NiftyNetModelZoo#2 branch for YAML files

[dzhoshkun]

• overridden methods with default parameter values cause problems (see above commit, and also fix a369e3e if applicable)

[dzhoshkun]

• ConfigParser.items returns a list of tuples, not a dict