Implemented new getMultiple pattern for all instances in Block: dataArrays, dataTags, sources, simpleTags
Implemented new getMultiple pattern for the one instance in DataArray: dimensions
Implemented new getMultiple pattern for one instance in DataTag: references. To achieve this a new 'getReference(size_t index)' was added that accepts an index instead of an id and that checks whether block has the referenced data array.
Implemented new getMultiple pattern for last instance in DataTag: representations.
Comeback of findSources in Block, implemented by using Sources::findSoures. This implementations allows duplicate findings however.
Implemented new getMultiple pattern for all instances in File: blocks, sections.
Implemented new getMultiple pattern for all instances in Section: sections, properties.
Implemented new getMultiple pattern for all instances in SimpleTag: references, representations.
(Therefore in analogy to DataTag a "getReference(size_t index)" was implemented)
Fixed errors appearing in "File" test case
Merge pull request #75 from balint42/master

Implemented new getMultiple pattern for all instances in Block

This was quite allot: thanks. 
As far as I can tell everything is OK.