Don't shadow builtin min and max in quantifier-related code

[akosthekiss]

Both the tools (the processor, the codegen template) and the runtime (the models) contain code that deals with quantifiers. Quantification has lower and upper bounds, which are traditionally named min and max. However, having min and max as function arguments (like in models) shadows the Python builtins. To avoid this and make the builtins available, min is renamed to start while max is renamed to stop all around the code base (terminology borrowed from the range Python builtin).

[coveralls]

coverage: 88.246%. remained the same when pulling 4fe60a5b67af1b84ea48330206f92a04a767d009 on akosthekiss:quant-start-stop into bc26b565094f3fd37d6b6f59a2e84234b6c5200b on renatahodovan:master.