update STIR requirements to v6.3

[KrisThielemans]

With STIR 6.2, all Hessian tests will/should fail.

I think the easiest thing will be to require STIR 6.3 in the find_package statement and to update the SIRF-SuperBuild to latest hash once UCL/STIR#1651 is merged. If people complain, we can always put in some code to allow 6.2, but throw explicit errors when the Hessian code is called (due to #1343 or UCL/STIR#1648)

Originally posted by @KrisThielemans in #1346 (comment)

[KrisThielemans]

@paskino this really needs an issue in the SIRF-SuperBuild as well

[KrisThielemans]

new STIR master hash is available!