Allow user to set URL/TAG for STIR, Gadgetron, SIRF, ISMRMRD
Allow user to set URL/TAG for STIR, Gadgetron, SIRF, ISMRMRD

The user can now override the default configuration by setting the values of
<package>_TAG <package>_URL for each of STIR, SIRF, Gadgetron and ISMRMRD packages

Corrects a bug associating default URL to TAG.

CCPPETMR/SIRF-SuperBuild,CCPPETMR/SIRF-SuperBuild