Improve array indexing consistency

[jquartel]

This is an extension of #5361.

In grel, if I have an array a which is ["x","y"], say, then I can index a with values -2, -1 ,0 , and 1 with no error and the logic, for the negative values, has a well-defined (and documented) meaning in terms of referencing from the end of the array (i.e. a[-1] is "y" in this instance). And if I go out of bounds with a positive index, such as a[2], then this results in an out-of-bounds error. However, if I go out of bounds with a negative index, such as a[-3], then this just returns the first element of a and never gives an error. I suggest it should give an error.

The use of a second indexing argument to describe a range (e.g. a[0,2]) is also inconsistent, as described in #5361. If we call that second argument M, then in general the last element of the range is the (M-1)th, where negative values of M reference the array from the end as per the first argument (e.g. a[-2,-1] would give "x" in the example array above). However M=0 is an exception since it does not result in the last element of the range being the -1th (i.e. the first from the end) - instead you always get an empty array when M=0 (so a[-2,0] gives [] instead of ["x","y"] which I think would be more consistent). Moreover, there is no out-of bounds condition with the second argument. If M > length(a) then it behaves like length(a) (e.g. a[1,5] gives the same result as a[1,2]), and if M is less than -length(a) then it behaves like -length(a) (which always results in an empty range). Would it not be more consistent to produce bounding errors with the second argument as for the first?

[tfmorris]

I think the inconsistent error behavior for a[-3] is a bug.

The behavior of the second argument could probably be improved, but it may make sense to split it into its own issue so that a proposal can be developed and carefully reviewed.

[jquartel]

#5361 is focussed on the second (ranging) argument, so I think it's already an issue on it's own.