use a generic interface for predicates

[jlapeyre]

I started a repo IsApprox.jl with the goal of providing a flexible and generic interface for predicates that might require tolerance. I noticed that things like ishermitian and isposdef are frequently reimplemented in a more-or-less ad hoc way, and wanted to avoid doing the same thing myself.

You may want to comment on the design. If it turns out to be useful, you might eventually depend on it. Some parts of it would actually work well as a drop-in replacement in Base and LinearAlgebra. But, it would have to be demonstrated to be useful and solid first.

[matthewware]

This looks really nice! I have been getting a bit tired of fixing real data interfaces with ad hoc tolerances. Let me take a dive and play with he code a bit.

[jlapeyre]

It should be registered in 3 days. I'll make an announcement on discourse.