feat: Introduce short-term rating translation strategies

[a-vester]

This feature request is a consequence of a lively discussion.

pyratings current approach to short-term ratings is to make them comparable across rating agencies (same philosophy as for long-term ratings). This is a pre-requisite in order to compute an average rating on a portfolio basis, where different securities have ratings from different agencies.
However, compared to long-term ratings, there is a significant difference. Different rating agencies use a different number of ratings. For example, while Moody`s uses four short-term ratings, S&P and Fitch use eight, and DBRS uses ten. This makes it generally somewhat difficult to compare ratings between different rating agencies.

Moody’s	S&P	Fitch	DBRS
P-1	A-1+	F1+	R-1 (High)
P-2	A-1	F1	R-1 (Mid)
P-3	A-2	F2	R-1 (Low)
NP	A-3	F3	R-2 (High)
	B	B	R-2 (Mid)
	C	C	R-2 (Low)
	D	D	R-3
	SD	RD	R-4
			R-5
			D

There's another issue with short-term ratings: All rating agencies have some overlaps in their rating scales. Why is that and what is it good for?

Short-term ratings are highly dependent on long-term ratings. However, they take into account the short-term prospects of an issuer from a liquidity perspective, i.e. the short-term rating may be better or worse than what could be implied by simply translating the long-term rating. The table below shows the possible variations.

For example, an issuer rated A3 would usually get assigned a short-term rating of P-2. However, if the liquidity prospects are better than what a P-2 rating would imply (e.g. the issuer might have some guaranties from other entities), a P-1 rating might be assigned.
On the other hand, an issuer rated A2 would normally get assigned a short-term rating of P-1. However, if the rating agency concludes that the liquidity prospects are worse than what a P-1 rating usually implies, the issuer might get assigned a rating of P-2.

These overlapping areas are a concern. Let's say you have a long-term rating score of 6, what's the correct short-term rating? - The answer is: You can't tell. It could be P-1 or P-2 (in terms of Moody's). Having said this, P-1 is still the most likely outcome.
Anyway, at that stage, the user has to make up his mind and take a decision. Remember, there's no right or wrong, just different ways of getting the job done.

There could be three distinct strategies in order to translate a long-term rating score into a short-term rating.

• Strategy 1 (best):
  You always choose the best possible short-term rating. In the case of a rating score of 7, that would translate into a short-term rating of P-1.
  That's the optimistic approach.
• Strategy 2 (base-case):
  You always choose the short-term rating that a rating agency would usually assign if there aren't any special liquidity issues (positive or negative). In that case, a rating score of 6 translates into a P-1 rating, while a rating score of 7 would translate into a P-2 short-term rating.
  That's the base-case approach.
• Strategy 3 (worst):
  You always choose the worst possible short-term rating. In the case of a rating score of 6, that would translate into a short-term rating of P-2.
  That's the conservative approach.

The default strategy should be strategy 2 (base-case) as it probably is the best fit for most situations.