Added population AIF as optional input parameter.

See "Experimentally-derived functional form for a population-averaged
high-temporal-resolution arterial input function for dynamic
contrast-enhanced MRI." by Parker et al, MRM 2006.
Use longFlag instead of flag.
Addressed PR comments.

Restored some original variable names.

Tightened restriction on when AIF mask is used, only when NOT using
either population aif or prescribed aif.
Addressed PR comments.

Improved one variable name.
Removed one unnecessary call to compute_bolus_arrival_time.
Removed unnecessary SetPopulationAIF macro.
Merge pull request #20 from QIICR/PopulationAIF

Population aif