Proposed resolution of non-unique IDs in GMSH format.
Add solving of non-unique IDs in GMSH format.

- add multi index member only for region IDs to RegionDB
- add exceptions for non-unique region IDs
- create RegionDB::find method for search by ID parameter
- small optimizations of RegionDB class (naming of types, method etc)
Modify of exception of non-unique region.

- use only input exception
Modify RegionDB::add_region method.

- remove check dimension consistency
- add code for adding region with undefined dimension
Finish of implementation of nonunique region id.

- fix error in add_region method
- add unit test
Optimization of RegionDB::add_region methods.

- create separate private methods for insert region, update dimension
and testing of existing region.
- use these methods in add_region methods
Fix error in RegionDB.
Remove boundary parameter in add_region method.
Fix error in insert implicit boundary region.
Merge branch '1.8.0_master' into DF_nonunique_id
Merge branch 'DF_nonunique_id' into 1.8.0_master