Implement `BillObject` class methods (Mutators)

Implement mutator methods inside Models/BillObject.swift

[x] mutating func clearEntries()
[x] mutating func setTitle(newTitle: String)
[x] mutating func setDate(date: Date)
[x] mutating func setTaxRate(tax: Double)
[x] mutating func setInitiator(initiator: Int)
[x] mutating func setParticipants(participants: [Int])
[x] mutating func removeParticipant(at: Int)
[x] mutating func addNewEntry(entry: EntryObject)
[x] mutating func addNewEntry(entryTitle: String, participants: [Int], value: Double, amount: Int, withTax: Bool)
[x] mutating func removeEntry(at: Int)

cs130-w21 / dots-ios