[lehni]

Closes #537

Adds a static Model.getGraphSettings(relationPath, options, operation) that resolves the strict mode settings of a relation path from the options of a strict insertGraph() or upsertGraph() call, with the same code as the writer (GraphOptions.getSetting()):

Person.getGraphSettings('movies.actors', options, 'upsert');
// → { removed: 'preserve', added: 'relate', kept: 'patch', created: 'insert' }

• relationPath is a string like 'movies.actors' or an array of relation names. An empty path gives the root settings.
• operation is 'insert' or 'upsert', as the defaults differ between the two.
• Relations that the model doesn't have throw, so a misspelled path doesn't get the defaults. So do non-strict options, unknown operations and options of the other method.
• GraphOptions.getSettings() returns all four settings of a path. The typings return GraphSettings and InsertGraphSettings, typed per operation.