#126 errors in syntheticcdo.cpp

[leibniz777]

There should be made the following corrections:

1.) Implementation of Real SyntheticCDO::protectionLegNPV():
The last return value should be protectionValue_ instead of premiumValue_.

2.) Implementation of Rate SyntheticCDO::fairPremium ():
If the runningRate_ is set to zero, the premiumValue_ would be zero. This means that the return value is an error value. Wouldn't it be better to introduce the concept of a premium bpv and to use this value to calculate the fair premium?

[leibniz777]

I also suggest to implement a function that gives the upfront premium value similar to the function premiumValue() that gives the running premium value.

[Luigi Ballabio]

Moved to https://github.com/lballabio/QuantLib/issues/365