Index-calculator

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* optionally: calculate SCD with water equivalent input
* replace '.' with '-' in directory structure and output file names
* new climate indicators:

* DSn: total number of days in dry spells
* WSn: total number of days in wet spells
* CSx: maximum length of cold spell
* CSn: total number of days in cold spells
* HSn: total number of days in hot spells
* HWn: total number of days in heat waves
* LFD: number of late frost days

* metadata: set mm/day to mm
* fixing v0.7.3 and v0.7.4 with xclim>=0.44
* redo v0.7.5 (if dataset frequency is not equal input frequency, convert it; adding hourly test data)

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* outsourcing test data to github repository https://github.com/ludwiglierhammer/test_data

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* if dataset frequency is not equal input frequency, convert it
* adding hourly test data
* calcualte climate indicators for ERA5 data
* input variable for WI is tasmin not tas

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* rename DSP to DSf according to other spell indicators
* rename HW to HWx according to other spell indicators
* new indicator: DSx, HWf
* rename tables/indices.json to tables/metadata.json
* expand some docstrings and rework metadata
* those changes are not implemented in xclim yet
* as soon as this is done index_calculator goes v0.8.0

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* rework metadata and make it more flexible regarding threshold values
* more parameter information in cliamte indices docstrings
* adjust parameter names to xclim
* set threshold for dry day incides from 0.1 mm/day to 1 mm/day
* set threshold from snowfall rate to liquid water equivalent snowfall rate with SD, Sint and Sfreq -> those changes are not implemented in xclim yet, PR will be accepted with xclim>=0.45

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* new indicators implemented:

* FG (mean daily wind speed)
* FGn (minimum od mean daily wind speed)
* FGx (maximum of mean daily wind speed)
* FX (mean of maximum daily wind speed)
* FXn (minimum of daily maximum wind speed)
* FXx (maximum of daily maximum wind speed)