Add a macro @throttle

[mcabbott]

We're discussing removing callbacks in favour of just writing a loop.

One nice thing you can do with callbacks is cb = throttle(mysave, 60) to save roughly once a minute, etc. This PR adds a macro that lets you write @throttle 60 mysave() inside your for-loop. It should accept an arbitrary block of code, which may depend on local variables, e.g.:

for i in 1:100
    g = ...
    @throttle 0.5 begin  # macro creates fun(save, m, g, i) = begin ...
       save("file", m, g, i)
    end
    ...

(It re-uses the function throttle, and has to eval some things to happen before the loop it's in.)

PR Checklist

• Tests are added
• Entry in NEWS.md
• Documentation, if applicable

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 71.34%. Comparing base (7be1ca7) to head (e8572ab).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/utils.jl	20.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2120       +/-   ##
===========================================
+ Coverage   34.55%   71.34%   +36.78%     
===========================================
  Files          31       31               
  Lines        1904     1965       +61     
===========================================
+ Hits          658     1402      +744     
+ Misses       1246      563      -683     

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