Rename variables before processing

[jzhu20]

In results from current Health-GPS main branch, a prefix "mean_" is added to indicate mean values of variables, which is inconsistent with the variable names in R scripts. Therefore, we need to remove "mean_" before processing data.
Add the following commands to https://github.com/imperialCHEPI/healthgpsrvis/blob/main/R/data-process.R Line 1.

## Renaming variables if necessary, to remove prefix ##
names(data) <- sub("^mean_", "", names(data))