API: New registry method to gather all patterns in a package as a Group

[istvanrath]

As per https://trac.inf.mit.bme.hu/VIATRA/wiki/IncQuery2/Meetings/20120611:

• package split across multiple projects is allowed 0.6
  • matcher factory registry will introduce a method to create/return a group based on a package FQN (caveat: works only with registered patterns)

This is an additional task related to #14.

[abelhegedus]

The current implementation has two questionable additions to the specification:

• it supports package subtree inclusion, so each pattern that is in a subpackage of the given package is put into the group. This is accessed through a different method from strict pattern groups.
• it stores already calculated groups and can return them without reinitialization. These stored groups are kept up-to-date in the rare cases when new matcher factories are registered.

Any of the above behaviour may seem unwanted/erroneous/smelly to you. Discuss! (or close the issue :D)

[istvanrath]

The only thing I would like to point out is that I thought it would be a good idea to introduce our own class for PatternGroups, instead of using plain Sets. The advantage would be that down the line, where we might need to add additional features to patterngroups, such changes would not break the API.

[abelhegedus]

I agree that an own class for groups is a good idea. However, I believe that it should be handled apart from the matcher factory registry. Actually, it should be each pattern group's responsibility to refresh their included patterns by calling the registry and the registry should remain independent of group objects and work only as a way to retieve patterns to create a group.

[abelhegedus]

Ready for use in #182