Resampling in getobs causing an overwrite of measurement error if it is named '_variability'

[rt17603]

Original report by Rebecca Ward (Bitbucket: [Rebecca Ward](https://bitbucket.org/Rebecca Ward), GitHub: rebeccawardbristol).

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My Arctic data was processed with the name ‘ch4_variability’ for the measurement error. When retrieving data with getobs and using the average input, it is assumes the measurement error is named ‘…repeatability’ (line 491 in read.py), then resamples that variable and creates a new variabile called 'mf_variability' which contains the standard deviation within the resampling period.

My variability values were therefore being overwritten. For now I have changed the name of my measurement error in the Arctic data to ‘ch4_repeatability’. Either need to prevent variability value from being overwrtitten or specify that it cannot be called ‘ch4_variabiilty’ in the obs tutorial.