Remove inner Coordinate struct from TableMultiVariableLookup

[#73049874]

https://www.pivotaltracker.com/story/show/73049874
@kbenne this allows you to repeat the same point more than once, is that something you want to prevent in the API?

Having same x and same y is maybe ok (but not ideal):
[0,0,0]=>1.0, [0,0,0]=>1.0, [0,1,1]=>2.0

Having same x but different y is bad:
[0,0,0]=>1.0, [0,0,0]=>3.0, [0,1,1]=>2.0

Is there a unit test for this case?  The bool return value indicates to me that this function should do some checking