hstore add function for float values

- added adeven_float_add => hstore double values will be summed with a precision of 0.0001
- renamed welle_add to adeven_long_add
- updated Makefile and postgres_agg.sql accordingly to other changes

note:
- adeven_float_add uses internally double not float values as i found a function name like adeven_double_add confusing

- i will do the refactoring and drying steps later, as they are more complicated as i first thought