Support for providing both `fields` and `modelClass` to a Store config

[matsbryntse]

    store : {
        fields = [
            { name : 'restaurant' },
            { name : 'location' },
            { name : 'review' },
            { name : 'response', type : 'formula' },
            { name : 'foodItem', type : 'formula' },
            { name : 'sentiment', type : 'formula' },
            { name : 'notes', type : 'formula' }
        ],
        modelClass : SpreadsheetRow,