Add Findcppgsl.cmake, 1.0 branch (2018.10.11.)

!364 (merged) made me realise that Findcppgsl.cmake was never added to the 1.0 branch.

This is just cherry-picking the two commits from master that add this module.

AE Build SUCCESS
Build logfiles are available at Jenkins [AE-MERGE-REQUEST #578]

merged

mentioned in commit 1fa5a39b