Deprecate CamelCase convenience for snake_case constructors

[mkitti]

Specifically, deprecate LinearInterpolation, ConstantInterpolation,
and CubicSplineInterpolation in favor of linear_interpolation,
constant_interpolation, and cubic_spline_interpolation, respectively
since these are merely functions and not types.

Put another way:

julia> Dict([
           :ConstantInterpolation => :constant_interpolation,
           :LinearInterpolation => :linear_interpolation,
           :CubicSplineInterpolation => :cubic_spline_interpolation
       ])
Dict{Symbol, Symbol} with 3 entries:
  :CubicSplineInterpolation => :cubic_spline_interpolation
  :LinearInterpolation      => :linear_interpolation
  :ConstantInterpolation    => :constant_interpolation

[codecov]

Codecov Report

Merging #501 (e9fe6c7) into master (4bd7af8) will decrease coverage by 0.18%.
The diff coverage is 100.00%.

❗ Current head e9fe6c7 differs from pull request most recent head a4226c0. Consider uploading reports for the commit a4226c0 to get more accurate results

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
- Coverage   87.10%   86.91%   -0.19%     
==========================================
  Files          28       27       -1     
  Lines        1845     1819      -26     
==========================================
- Hits         1607     1581      -26     
  Misses        238      238              

Impacted Files	Coverage Δ
src/Interpolations.jl	79.52% <ø> (+3.60%)	⬆️
src/deprecations.jl	2.56% <ø> (ø)
src/iterate.jl	100.00% <ø> (ø)
src/convenience-constructors.jl	100.00% <100.00%> (ø)
src/gridded/gridded.jl	96.82% <0.00%> (-1.59%)	⬇️
src/b-splines/b-splines.jl	90.00% <0.00%> (-0.10%)	⬇️
src/lanczos/lanczos.jl	100.00% <0.00%> (ø)
src/gpu_support.jl

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