Add Previous and Next for Constant interpolation

[judober]

Fix #511

I thought of adding Constant{Mode}() to the docstring of interpolate but was not shure if this is apropriate. Thereby I noted that these docstring have different entries which seem inconsistent/incomplete. E.g. once there is BSpline(NoInterp()) and once only NoInterp(). Also these do not reflect the posibility of using Gridded.

[codecov[bot]]

Codecov Report

Merging #512 (1de4c3f) into master (e6d076a) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #512   +/-   ##
=======================================
  Coverage   87.10%   87.10%           
=======================================
  Files          28       28           
  Lines        1846     1846           
=======================================
  Hits         1608     1608           
  Misses        238      238           

Impacted Files	Coverage Δ
src/b-splines/constant.jl	100.00% <ø> (ø)

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[mkitti]

I think expanding the doc string for Constant is sufficient, but I will think about discoverability more later.

[mkitti]

Documentation preview: https://juliamath.github.io/Interpolations.jl/previews/PR512

[mkitti]

I'm not sure why the preview is not working. I'll investigate later.

[judober]

Anything left to do here?

[mkitti]

Looks good to me