Change default tolerances for testers based on eltype

[sethaxen]

The default tolerance of atol=1e-9, rtol=1e-9 for the isapprox tests seem like they might be too stringent for 32-bit checks. Perhaps we should have a utility method for settings these tolerances based on the float eltype of the inputs?

[oxinabox]

yes, in general FiniteDifferences.jl doesn't work well on Float32.