Fix/lua no funcs known (#15)

* lua fixes:

- fix lua function calling, didn't work in the last
releases (regression)
- add lua funcs which don't modify the stack (for converters etc)
- added better lua examples

README.md

@@ -225,10 +225,15 @@ the `register()` function to register your functions to the
 calculator. This function takes these parameters:
 
 - function name
-- number of arguments expected (1,2 or -1 allowed), -1 means batch
-  mode
+- number of arguments expected (see below)
 - help text
 
+Number of expected arguments can be:
+
+- 0: expect 1 argument but do NOT modify the stack
+- 1-n: do a singular calculation
+- -1: batch mode work with all numbers on the stack 
+
 Please [refer to the lua language
 reference](https://www.lua.org/manual/5.4/) for more details about
 LUA.