review Material API

• let users build up their own materials library (similar to instruments library) (ex #155 (closed))
• #53 compute SLD from chemical formula and energy
• Hold IMaterial in unique_ptr, instead of pimpl (Material -> BaseMaterialImpl) (ex #294 (closed) a)
• Core classes should not hold name. On the other hand, material name makes sense in GUI (ex #294 (closed) b)
• #561 internal SLD units are inconsistent
• provide color for use in real-space view #349
• use through shared instead of unique pointers, especially for use in real-space view #349