Add polymorphic relationships about rekord HOT 2 CLOSED

• A model can have no special fields, but have a polymorphic model point to it.
• A model can be polymorphic and contain a discriminator and another model's keys.
• A polymorphic model is typically tightly related to it's parent, so when it's parent is removed it should be removed by default.

from rekord.

Discriminated relationships will work by injecting discriminator functions into a relation - this will avoid this logic everywhere:

if ( this.discriminated ) {
   // do something
} else {
  // do somethine else
}

The following functions need to be created in NeuroRelation to make this possible:

• parseModel(input, remoteData)
• createCollection()
• listenToModelAdded(callback)
• grabModel(initial, callback, remoteData)
• grabModels(initials, callback, remoteData)

The existing functions need to be modified for discriminated:

• isRelatedFactory(model)

A few more things:

• Move hasFields out of NeuroDatabase

from rekord.