Flexible storage for image metadata

[mkuemmel]

FITS keywords are currently stored in structs "boost::variant". "boost::variant" allows a set of types, and when reading the value the correct type must be known and addressed.

This results in problem for metadata that are double in SE++ but the user stored them in a different type (e.g. int), since the type conversion from "boost::variant" is cumbersome.

The problem appear in #405, and #406 offers a temporary, but not elegant fix.

Also other keywords could be affected, and it might be better to find a permanent fix.