Consistency Checks: Coupon Dates <=> Coupons, Coupon Periods <=> Spreads

[Stefan]

• Throw an error message if the number of coupons is not equal to the number of coupon dates.
• Avoid kernel shutdown if coupon dates, but no coupons (i.e. empty list) are porvided.
• Throw an error message if the number of spreads is not equal to the number of coupon dates minus one:
  len(Spreads) == len(Coupon_Period_Dates) - 1.

[Stefan]

to be tested by Stefan