Fixed warn deprecation + tests raising warnings

[fguevaravas]

It seems warn has been deprecated since Julia v0.7 and we should be using the @warn macro instead. warn is used in two locations:roots and Splined2D. This PR also adds tests for warnings being issued in the following.

1. in roots when there are more roots than the maximum number of roots
2. in Spline2D when some internal least squares problem that is solved by the Fortran code is rank deficient. This example was harder to come by and required adjusting s carefully. Since rank-deficiency could be compiler/processor/platform dependent, it may be better to remove this test (but still keeping the changes in src/Dierckx.jl)

[kbarbary]

Thanks!

Looks like one failing build on Julia 1.3, windows 32-bit:

Expression: Spline2D(x, y, z, s=85)
  The required storage space exceeds the available storage space:
  nxest or nyest too small, or s too small. Try increasing s.

Not sure why this would fail just on this one platform. Should we remove this test?

[fguevaravas]

Ok, I just removed the test which is not platform stable. The tests should pass now.

[fguevaravas]

It looks like there are still errors in the tests, but these are due to reasons unrelated to this PR, as far as I can tell.

[kbarbary]

Thanks!