Package index: Euribor and Euribor365 class with months and weeks slight optimisation for performance

[Henry Freedman]

All the Euribor and Euribor365 class with months and weeks should be refactored to use final objects as Periods and not create a new one for every client.

BEFORE:
public class Euribor10M extends Euribor {

    public Euribor10M() {
      this(new Handle<YieldTermStructure>());
    }

    public Euribor10M(final Handle<YieldTermStructure> h) {
        super(new Period(10, TimeUnit.Months), h);
    }

AFTER:
public class Euribor10M extends Euribor {
 public static final Period TENOR = new Period(10, TimeUnit.Months);

    public Euribor10M() { this(new Handle<YieldTermStructure>()); }
    public Euribor10M(final Handle<YieldTermStructure> yieldTermStructure) { super(TENOR, yieldTermStructure); }
}

[Henry Freedman (henry-6th)]

Optimise also the sub package index.ibor