ENH: Update the internal index of TemplateFlow

[oesteban]

This PR introduces templateflow.conf.update() to allow for the update the TemplateFlow structure pointed by TEMPLATEFLOW_HOME.

It has a "local" operation mode (which uses the skeleton distributed within the installed package) to ensure the continuity of previous work on this line. This means that, when re-installing templateflow, the TEMPLATEFLOW_HOME will still be updated the same way it used to be.

This PR is limited, as it does not allow to point templateflow to a particular release of the archive. The main reason for that is that we would need to have a version controlled repository of skeletons (which does not exist yet).

Resolves: #37

[codecov[bot]]

Codecov Report

Merging #40 into master will increase coverage by 2.24%. The diff coverage is 98.43%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   85.44%   87.69%   +2.24%     
==========================================
  Files           5        6       +1     
  Lines         158      195      +37     
==========================================
+ Hits          135      171      +36     
- Misses         23       24       +1     

Flag	Coverage Δ
#api	87.69% <98.43%> (+2.24%)	:arrow_up:
#config	45.64% <75.00%> (+2.60%)	:arrow_up:
#datalad	77.94% <98.43%> (+4.53%)	:arrow_up:
#dls3	40.00% <4.68%> (-8.74%)	:arrow_down:
#s3	52.82% <39.06%> (-0.35%)	:arrow_down:

Impacted Files	Coverage Δ
templateflow/api.py	77.90% <ø> (ø)
templateflow/conf/_s3.py	97.82% <97.82%> (ø)
templateflow/conf/__init__.py	91.30% <100.00%> (-1.43%)	:arrow_down:

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[oesteban]

Sorry for the noise of adding you as a reviewer. I'll go ahead since I want to get this finished asap