Add support for <= and >= for Approx

[jbcoe]

closes #651

[jbcoe]

failing tests are unconnected with this change and addressed by #716

[onqtam]

looking at this PR - isn't it possible to also implement less-than and greater-than - like this:

friend bool operator< (double lhs, Approx const& rhs) { return lhs < rhs.m_value && lhs != rhs; }
friend bool operator< (Approx const& lhs, double rhs) { return lhs.m_value < rhs && lhs != rhs; }
friend bool operator> (double lhs, Approx const& rhs) { return lhs > rhs.m_value && lhs != rhs; }
friend bool operator> (Approx const& lhs, double rhs) { return lhs.m_value > rhs && lhs != rhs; }

by using && and not-equals instead of equals?

[jbcoe]

@onqtam I think you are right. I'm not sure I'd find <, > useful for Approx. Do you have a use-case in mind?

[onqtam]

not really - but just for completeness' sake

[jbcoe]

REQUIRE(x < Approx(2.3));

seems a bit odd to me. I'd be reluctant to add your functions for completeness' sake without a motivating use case.

[horenmar]

I am against adding those.