Initiated NSDFReader.
Added function to get uniform data row by source id and field name.
`timeunit` attribute name changed to `tunit` to match proposal.
NSDFReader - passing test to get uniform data.
Updated docs to include nsdf reader.
Passes unit tests for getting uniform, nonuniform and event data for
ONED dialect.
Now NSDFReader can read all three kinds of data.
Merge pull request #22 from subhacom/subhasis

NSDFReader basic implementation