remove stheno; add AbstractGP surrogate

[vikram-s-narayan]

1. Added @safetests with @time macro in runtests.jl - this provides a nice output of how long each test takes to run
2. Removed SthenoKriging and all references to that as it was breaking after the update from stheno 0.6 to 0.8; also, after consulting the folks at AbstractGPs.jl and Stheno.jl, it was decided that it's better to go with AbstractGPs.jl
3. Added a new kind of surrogate model for Gaussian Process regression called AbstractGP which is a simple wrapper around AbstractGPs.jl
4. Also added Flux and PolyChaos to dependencies. This gets rid of the warnings we have been getting.

This PR is a fix for Issue #318.
It is also a fix for Issue #308.

[codecov]

Codecov Report

Merging #321 (4595aa4) into master (74645db) will decrease coverage by 1.11%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
- Coverage   80.34%   79.23%   -1.12%     
==========================================
  Files          19       20       +1     
  Lines        2305     2331      +26     
==========================================
- Hits         1852     1847       -5     
- Misses        453      484      +31     

Impacted Files	Coverage Δ
src/Surrogates.jl	30.76% <ø> (-2.57%)	⬇️
src/AbstractGP.jl	90.90% <90.90%> (ø)
src/Radials.jl	87.85% <100.00%> (ø)
src/SthenoKriging.jl	0.00% <0.00%> (-89.75%)	⬇️
src/Wendland.jl	100.00% <0.00%> (ø)
src/Lobachevsky.jl	97.19% <0.00%> (+0.02%)	⬆️
src/Optimization.jl	73.30% <0.00%> (+0.62%)	⬆️

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[ChrisRackauckas]

This just needs a docs update. Everything else looks good.