improve riskmodel

[x0range]

riskmodel.py is messy, has boiler plate code, and is difficult to read. Should be redone in a clearer way. Such that it does two things:

• give estimates about whether the company may fail in a catastrophe with the current capital and portfolio
• give estimates of the contribution of any one underwritten contract and any one offered but not yet underwritten contract to this
  This includes the balancedness of the portfolio.