Index-calculator

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* take coordinate attributes from input dataset

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* component name adjusments with HYRAS

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* some metadata corrections

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* new indices:

* HW: maximum length of heat waves
* GSS: start of growing season
* GSE: end of growing season
* FFS: start of frost-free season
* FFE: end of frost-free season
* RRm: mean daily precipitation
* RRYYp: precipitation percentile value

* rename RYYp to RYYpABS and RDYYP to RYYp according to ICCLIM
* optional argument perc woth percentile indicators

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* time controlling to pyhomogenize

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* new index (WI): number of winter days (tas<-10°C)
* use pyhomogenize>=0.2.9
* write time and time_bnds to float