preprocHDSS: Pre-process raw HDSS data
In alexploner/R.HDSS: Process HDSS demographic data from the INDEPTH project

This function cleans up some common data variants that appear in raw HDSS data files.

1	preprocHDSS(x, checkVA = FALSE)

`x`	a data frame with raw HDSS data
`checkVA`	A logical flag indicating whether to test for the presence and validity of verbal autopsy data.

The function undertakes the following modifications:

empty dates are set to NA;
dates get stripped of trailing time stamps and converted to Date-variables, regardless of the separator used;
Sex is coded as an f/m factor;
for verbal autopsy variables, blanks are replaced with NAs;
cause of death codes are matched against the canonical list of approved causes (based on WHO standard).