Topic/plat 6816

* Implemented **VariableNotionalProvider** as a subclass of **NotionalProvider**. 
* The new class can be constructed with a set of dates and notionals, or just notionals. 
* In the latter case the date set is added in **AbstractAnnuityDefinitionBuilder**, that is, after computing schedule in **FixedAnnuityDefinitionBuilder** or **FloatingAnnuityDefinitionBuilder**.
* Tests for these changes are **VariableNotionalProviderTest**, **FixedAnnuityDefinitionBuilderTest** and **FloatingAnnuityDefinitionBuilderTest**.
* Amortizing swap example is added to **SwapInstrumentsDataSet** and **SwapCalculatorE2ETest**.