output for config value
category to get key from
key whose value is to be got
if the specified key does not exist
TODO: perhaps we should discuss removing the other version of getStrict(), above? It seems a little bit confusing having both methods, and I feel this version is more convenient to use.
Alternative form strict config value getter, returning the retrieved value via a reference. (The advantage being that the template type can then be inferred by the compiler.)
Template can be instantiated with integer, float or string (istring) type.
Usage Example: