BREAKING: Unexport gradient and friends

[tomasaschan]

This is another take on solving the same problems as #222, but instead of exporting a new function gradient, I have removed it (and its friends) from the list of exported functions.

By qualifying all usages in the test suite as Interpolations.gradient, they now pass on 1.0! 🎉

However, I'd like to figure out a way to make this transition a little kinder to the users. Is there a way to deprecate usage like

using Interpolations
itp = ...
gradient(itp, ...)

while still allowing

using Interpolations
itp = ...
Interpolations.gradient(itp, ...)

?

If not, maybe we just have to accept that this is part of a somewhat painful transition to 1.0.

Edit: I've incorporated #223 here too, because making it work on Julia 1.0 is the main motivation behind this PR. Doesn't matter to me if we merge #223 first and then this (which will effectively rebase that commit away) or close #223 and take the change from here.

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Side note: It's not so weird to me after making these changes that we have problems with Hessians (se e.g. #122 and #181). There's not a single mention of hessian in the test suite...

[codecov-io]

Codecov Report

Merging #225 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #225   +/-   ##
=======================================
  Coverage   81.73%   81.73%           
=======================================
  Files          27       27           
  Lines        1002     1002           
=======================================
  Hits          819      819           
  Misses        183      183

Impacted Files	Coverage Δ
src/Interpolations.jl	90.32% <ø> (ø)	⬆️

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[sglyon]

I agree with the notion of Interpolations not "claiming" the function gradient (can't remember which issue I saw that comment in) and not exporting the function is a good way to accomplish that.

I think users have two options:

using Interpolations
itp = ...
Interpolations.gradient(itp, ...)

or

using Interpolations
using Interpolations.gradient
itp = ...
gradient(itp, ...)

As long as gradient has not been loaded/imported by another used package already.

If we are content with that outcome I think this PR is good to go!

[timholy]

If you're eager this can be merged, but I already have this on my local branch and lots of other work too.

If you do merge, I would argue against tagging 0.7/1.0 compatible just yet. I am getting very close (<1day?) to having a full rewrite done.

[tomasaschan]

It doesn't matter much to me - I just reacted to the fact that quite a lot of people seemed eager to get something 1.0-compatible out there, judging by the number of recent PR:s trying to accomplish this. With this, there is a git hash people can check out if they need something now, but I see no reason not to wait for your larger batch of changes in general.

[timholy]

Superseded by #226