staircase: make it simpler
We get the base dates from the current series value dates. For each of those we query one point, then concatenate the result. Conceptually much simpler to grasp. Bonus side effect: staircase discrepencies of formula vs alias extensions have disappeared. Performance implications: base: DELTA all value dates 0.9304628372192383 DELTA 1 day 0.10948061943054199 correctness: DELTA all value dates 1.174530267715454 DELTA 1 day 0.05773115158081055 So on the whole horizon we are a tad slower but there is an interesting speedup already in a well-bounded case. This is not bad.