Enhancement/select non equi sub

Adding non equi comparisons in the "sub" argument of select_ct (passing to data.table's i).

flex_sub <- function(x, sub) {
	x[eval(substitute(sub))]
}
test_quote(pts, allroads > units::as_units(200, 'm'))

TODO:

• check units and assume (+message) if missing OR add "assume units" arg
• ~~ check that columns exist?~~
• checks/tests: if list provided, if not an expression, check if expression is well constructed with deparse(sub)
• update man
• update vignette
• bump version
• update news