offset range lookup ("WITHIN"), get cells that are both LEFT and RIGHT

[mikeAdamss]

a common issue we have with databaker is that the api only supports absolute directions, so it can be DIRECTLY or CLOSEST in a given direction, but we have huge issues (and write a lot of nasty workarounds) for when the cells we want are in the middle (eg both LEFT and RIGHT, relative to different observations cells).

The solution would be a new keyword to search with an offset range.

Example:

HDim(cells, "Dimension1", WITHIN(-1, 1), ABOVE)

The -1 and 1 params above represent the offset to search, i.e one to the left through to one to the right, working upwards.

This pattern is illustrated, below, with the numbers showing the order which the lookup would search for a corresponding dimension value with these params.

	A	B	C	D	E	F	G
			7	8	9
			4	5	6
			1	2	3
				observation

this would allow us to cleanly extract data from a table presenting the data with the following (and painfully common) pattern.

	A	B	C	D	E	F	G
		2014				2015
	obs	obs	obs		obs	obs	obs

[mikeAdamss]

pattern should probably be WITHIN(LEFT=1, RIGHT=1), much more readable

[mikeAdamss]

have got this largely written, with mainly tests, sanity checking and maybe a few days on tweaking performance required. Will create a pr to kick this task off.

[mikeAdamss]

done