Reading graphs with generic ids
review fix: deleting quotation mark in readers
Minor fixes after review:

Implemented input errors handling.
Style fix.
pankaj review fixes
Division of sample graphs into smaller chunks

This commit enables concurrent read of sample graphs.
Merge pull request #75 from szymonm/SampleFilesDivision

Division of sample graphs into smaller chunks
Merge pull request #71 from szymonm/ReadingStringGraphs

Reading graphs with generic ids