Treatment of spreads in model

[open-source-modelling]

The treatment of spreads in different asset classes has to be defined and written into the examples. The initial proposal is:

For fixed income, spreads have 3 components.

1. The sector/country specific spread (constant) (spread_base)
2. Bond specific spread (constant) (spread_individual)
3. Stress spread (optional) (spread_stress)

For now, they are treated as a constant parallel shift. A further refinement could be to make it vary through time or tenor.
For new bonds (those issued in the "future"), the bond specific spread will be set to 0.

For equity, spreads have the same format, however most will be set to 0. This is meant to keep the methodology consistent and make it flexible for further development.

This spread has to be added to the base curve when discounting (but not capitalizing in case of equity). The argument is that we can split discounting that shows "riskiness" from growth of the company which implies a change in the intrinsic value.