Changes to tractography file timestamp handling

Changed the timestamp to be stored as an explicit std::map<string, string> member in the Tractography::Properties class, rather than a double.
New timestamps are now generated in the __WriterBase__::create() function, and any pre-existing timestamp in the Properties class is ignored at file creation.
Closes #57.
Non-template functions defined in a header should always be declared inline
Simplification of tck.tsf checking & validation
Further simplification of validation of tsf/tck files
Move timestamp handling back to DWI::Tractography::Properties class
Merge pull request #62 from jdtournier/tck_timestamp

Merge: tractography file timestamp handling