fix(frontends/lean/inductive_cmd): generate error for inductive datatype declarations that will produce an eliminator that can only eliminate to Prop
feat(kernel/level): improve is_geq procedure for universe levels

Now, it also returns true for

          (succ^k1 a)  =?=  k2

where k1 >= k2
feat(library/unifier): generalize method process_succ_eq_max_core
feat(kernel/level): improve universe level pretty printer

Example: produce `l+2` instead of `succ (succ l)`.
feat(frontends/lean): better default for atomic notation
fix(util/lean_path): change the default LEAN_PATH, a file in the current directory cannot shadow a library file, fixes #321