Fix and simplify Chronology

[patnr]

Contrary to the original ambition, the Chronology is among the ugliest elements of DAPPER.

• The ticker should be oriented around obs-analysis-cycles. The intermediate forecast times should be provided by a sub-loop/ticker.
• HMM.Dyn and HMM.Obs (operators) should take k (or ko) as parameter, not t.
• Should change the meaning of K (and Ko) from "last index" (which we get with -1 anyway) to "length of array". Fixes the current heavy use of K+1 and Ko+1. This issue is a hangover from when I started I was used to Matlab/Fortran (1-based indexing).
• Should have observation time 0 ? This would be nice coz then len(tto) == len(tt) when dko = 1. But the 0th cycle would then have two obs-times instead of one...
• BurnIn complicates everything. Simplify? Provide automatic averaging for custom-length time series?
• Last I checked I could change the dko of a Chronology without it raising an exception nor having the desired effect. BUG!