Allow more metadata (MHD) to be stored in datasets and store this to file when saving

[smistad]

Store as map: std::map<string, string>

Methods: getMetadata() getMetadata(name) setMetadata(name, value) setMetadata(metadata)

Other notes:

• Set metadata on assignment (image1 = image2)
• Set spacing
• Detect errors with size and type etc.
• Allow newlines in values
• Read all metadata when loading mhd file from disk