approximate float/number comparison assertions in eunit

[richcarl]

There should be macros in eunit for asserting that 1) a float (or any number) is within some epsilon from some other number (absolute error), 2) that the relative error is within some percentage, 3) that two numbers are "close". It is unclear if all of these are needed, or just 1-2, or even just 1.

References:

​http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm

​http://www.boost.org/doc/libs/1_35_0/libs/test/doc/components/test_tools/floating_point_comparison.html

​http://www.cs.otago.ac.nz/staffpriv/ok/software.htm (pcfpcmp)