This commit adds support for "offset" ROW windows, where the preceding and following
window bounds are allowed to have negative values.  This allows window definitions to
exclude the current row entirely.

Prior to this change, ROW-based windows *had* to include the current row, causing
`preceding` and `following` to support only non-negative values.  Additionally, the
inclusion of the current row would count against the `min_periods` check.

The following is an example of the new "negative" semantics.  Consider the input:
```c++
auto const row = ints_column{1, 2, 3, 4};
```
If the window bounds are specified as (preceding=3, following=-1), then the window
for the third row (`3`) is `{1, 2}`.
`following=-1` indicates a "following" row *before* the current row.

A negative value for `preceding` follows the existing convention of including the
current row.  This makes it slightly more involved:
  1. `preceding=2` indicates *one* row before the current row.
  2. `preceding=1` indicates the current row.
  3. `preceding=0` indicates one row past (i.e. after) the current row.
  4. `preceding=-1` indicates two rows after the current row.
Et cetera.

`min_periods` checks continue to be honoured as before, but the requirement for
positive `min_periods` is dropped.  `min_periods` only need be non-negative.

Signed-off-by: MithunR <[email protected]>

Also, removed prints.

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This now explains the semantics for negative window bounds.

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