code: public CurrencyDateCalculatorBuilder<E> currencyPair(final String ccy1, final String ccy2, final SpotLag spotLag) {
          this.ccy1 = ccy1;
          this.ccy2 = ccy2;
          this.spotLag = spotLag;
          return this;
      }
docs: This specialises the calculator to the given currency pair and the SpotLag (0, 1, 2). Given than some currencies can have different
  SpotLag depending on the business, this is something that the user will have to provide.
  @param ccy1
  @param ccy2
  @param spotLag
  @return the builder