Add axis operator

[jjg-123]

Scott's observation is that the built-in for_each function should intuitively allow for operating like a map functor (in functional programming languages, map applies an operator every element of a list). Since QDL has a very general concept of stem, this means that some way to communicate this needs to be done.

Therefore, a backtick, `, now can be used to restrict the for_each to a given axis. The idea is that at that axis, every entry for the stem is treated as a unit and simply passed. If

a. := [[1,2,3], [4,5,6], [7,8,9], [10,11,12]]
Then to loop over the 4 elements and apply your function f you would issue

for_each(@f, a.`0)

yielding

[f(a.0). f(a.1), f(a.2), f(a.3)]

cf.
for_each(@f, a.); // no axis specified

yields the 3x4 array

[ [f(a.0.0), f(a.0.1). f(a.0.2), ...]]