Reduce tolerance during function evaluation : use str instead of tuple for dict keys

[Balaji Pokuri]

Current implementation uses tuple to convert query point into dictionary key, resulting in a 16 digit precision in keys. It may be better to use limited precision string instead of a tuple. That can also remove unrealistic accuracy in search space.

[Balaji Pokuri]

A simple fix is through commit a5d67a9 -- check nearness before adding point into covariance matrix even if function is evaluated.

[Balaji Pokuri]

a5d67a9