Sort out Spectrum containers

Currently FiberTraceSet is a std::vector<PTR<FiberTrace>>. For a SpectrumSet I originally had the same concept, but I found that the only way to get rid of the compiler warning "Returning address of a stack object" (something like that) was to go from std::vector<PTR<Spectrum>> to std::vector<Spectrum>. Actually I now have an idea what I might have done wrong (probably created a shared pointer without the "new" in the argument list - should check that) so I think for consistency and to reduce copying I should probably change that back to std::vector<PTR<Spectrum>> and try again...)