MatEnv Change

[Jasonguo9019]

Changed matdbinfo.findDetMaterial to return shared pointer instead of raw pointer.

[brownd1978]

This looks really good Jason. I made one very minor suggestion, after you address that I will merge.

[brownd1978]

A target for the next PR will be replacing the std::string* in
std::map< std::string*, std::shared_ptr, PtrLess > _matList;
with just std::string. There's no good reason for this to be a pointer, but you might have to change other things to get it to work.

[brownd1978]

I see the CI is failing on Linux: it looks like you're missing a #include