fix calls to constructor

[SimonDanisch]

KDTree and LoessModel only accept concrete matrices, while loess and KDTree accept abstract matrices.
This PR makes sure to convert before calling the inner KDTree / LoessModel constructor. Fixes: https://github.com/MakieOrg/AlgebraOfGraphics.jl/pull/462 I'm not 100% sure why the converts are needed, since Julia usually does call convert(FieldType, field)` and it seems to work fine for certain abstractmatrices, but fails for e.g.:

Loess.loess(reshape(view(rand(4), 1:4), (4, 1)), rand(4))

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (9fd7d9d) 89.90% compared to head (f352518) 89.90%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #70   +/-   ##
=======================================
  Coverage   89.90%   89.90%           
=======================================
  Files           2        2           
  Lines         208      208           
=======================================
  Hits          187      187           
  Misses         21       21           

Impacted Files	Coverage Δ
src/Loess.jl	83.80% <100.00%> (ø)
src/kd.jl	96.11% <100.00%> (ø)

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