reti_filter_by_kind: filter reti data by kind of land
In rea-osaka/reti: Utility to handle real estate transaction price infomation data

This function filters data by kind of land. There are four kind of land; residence, commrercial, factory and unresidence.

1	reti_filter_by_kind(df, kind)

`df`	A data which was read from reti csvfile.
`kind`	String which contains Four character "R" or "C" or "F" or "U"; each character represents kind of land. There are four kind of land, residence, commrercial, factory and unresidence, which represent "R", "C", "F", "U", respectively.