Intermittent convergence estimate failures

[jagot]

The same set of data points can lead to different estimates of the convergence rate, which sometimes leads to tests failing:

• Failure: https://ci.appveyor.com/project/jagot/compactbases-jl/builds/33863946/job/4hwakx40umwhundm#L272
• Success: https://ci.appveyor.com/project/jagot/compactbases-jl/builds/33756833/job/u7xqcq2wugkb0a53#L272

It does not seem to be dependent on architecture or Julia version, e.g. a recent failure on Mac, nightly: https://travis-ci.org/github/JuliaApproximation/CompactBases.jl/jobs/701688381#L386

[jagot]

Provisionally fixed in a4e5a849e1cb244c6142cb8704e0166488146af5