Add tags to pipelines

[glatard]

So that pipelines can be searched in application catalogs.

Do we want to use a tag schema? Tags can be very diverse.
Add tags for bids-compliant pipelines.

To be discussed with @michaelkain and @Cbarillot.

[glatard]

Discussion (Jan 10):

• Yann: there must be somewhere an open part so that properties can be easily extended.
• Examples of tags: specific medical imaging processes, domain, package (FSL), license.

[cmaumet]

This was discussed on the CARMIN call on January 10th, 2017, cf. minutes.

[glatard]

Decision (Jan 10): we leave it completely open for now, once we have more examples we see what people put and may attempt to model that.

[axlbonnet]

The development concerning open tags has been done, they're called properties (see #63 ).
So this is done, and further discussion about restricting the properties from a list will be in #84