Enhanced height parser.

Based on taginfo, there are at least 137k heights with the value 2 m, 105k with value 3 m, 71k 6 m... The current function CleanNumeric handles simple strings and converts them to a number (SELECT cleannumeric('2.0'::text); -> 2::numeric).

The function should not replace CleanNumeric function, but be another function to deal with these non-numeric values (with m, m, ft, ", ' ",...)

The new function will be used for parsing heights for buildings, for mountain_peak, and for aerodrome_label layers.

One attempt was already discussed in #362.