Potentially unsafe additions/subtractions of InterestRate objects

[auto-differentiation-dev]

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InterestRate objects are often added / subtracted in the codebase without first checking if their conventions match. For example, there are several instances of code like this:

mu = riskFreeRate_->forwardRate(t0, t0+dt, Continuous).rate()
    - dividendYield_->forwardRate(t0, t0+dt, Continuous).rate();

This is potentially unsafe in cases where both rate objects use different conventions.

As @lballabio suggested in #1413 , it would be safer to implement a custom operator+ and operator- in InterestRate that first checks that conventions match and returns another InterestRate object.

[boring-cyborg[bot]]

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