Remove shared pointer behaviour

[OliverWoolland]

No description provided.

[jfecroftRSE]

I think I would do

inline void ROCrate::addEntity(const std::string& id, Entity entity) {

and

// Add an entity to the RO-Crate's entity register
entities_.emplace(id, std::move(entity));

It would end up with the same behaviour. Currently Entity is passed by reference and then copied into entities_. Passing by value makes it clear that the Entity stored in the crate is a separate object.

[jfecroftRSE]

Looks good. The example on the front page now also needs updating. Currently it makes copies of the root and rootData entities, then updates them which doesn't affect the ones in the crate

// Get the root metadata entity from the crate and set the description
//Entity root = crate.getEntity("ro-crate-metadata.json");
Entity& root = crate.getEntity("ro-crate-metadata.json");
root.set("description", "RO-Crate Metadata File Descriptor (this file)");

// Add name, description to the root data entity (./)
//Entity rootData = crate.getEntity("./");
Entity& rootData = crate.getEntity("./");
rootData.set("name", "Example RO-Crate");
rootData.set("description", "The RO-Crate Root Data Entity");