mathmore/GSLInterpolator: Warn about suppression of further warnings
mathmore/GSLInterpolator: Warn about suppression of further warnings

The previous logic was such that the warning about suppressed further
warnings never executed. This commit also improves formatting according
to the coding conventions.

Fixes: https://sft.its.cern.ch/jira/browse/ROOT-8572

gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root