Standardizing root location

[KasperZutterman]

@woutslabbinck suggested it could be useful to have a standardized root e.g. root.ttl implemented for all bucketizers.

For the bucketizer-basic this could mean the root contains relations to all sequential pages (1.ttl ... n.ttl).

[KasperZutterman]

Some considerations as discussed with @ddvlanck:

• For big LDESes the size of the root.ttl could grow substantially
• When using root.ttl as the root, in some bucketizations such as the bucketizer-substring this could cause collisions with strings containing 'root', for example, 'Rootmoerelaan'

Initial suggestions:

• allow for root pagination: root.ttl .. root-1.ttl .. root-n.ttl
• use root as the default root and allow the end user to change for avoiding collisions

[KasperZutterman]

Also add method getRoot() to return root location.
This can be used for publishing announcements, see : TREEcg/LDES-Action#57