added remove_at with its unit tests, ocamldoc and interface
Merge pull request #544 from UnixJunkie/batlist_remove_at

added remove_at with its unit tests, ocamldoc and interface
BatSeq: `min` and `max` throw `Invalid_argument`

Add to the documentation for `min` and `max` that they throw `Invalid_argument` upon an empty sequence.
Merge pull request #546 from nbraud/patch-2

BatSeq: `min` and `max` throw `Invalid_argument`
Add a few tests of BatAvlTree.make_tree

These test that the AVL tree height invariant is maintained during construction
and that an AVL tree contains the values put into it.
Merge pull request #548 from pminten/avltree-tests

Add a few tests of BatAvlTree.make_tree
batList: turn some Invalid_argument messages into private constants for robustness
new BatList.modify_at : int -> ('a -> 'a) -> 'a list -> 'a list
new BatList.modify_opt_at : int -> ('a -> 'a option) -> 'a list -> 'a list
Merge pull request #555 from gasche/modify_at

batList.{modify_at,modify_opt_at}: modify values at a given list position