revert change of FiniteGP constructor from #236 to avoid breaking downstream packages

[st--]

The

FiniteGP(f::AbstractGP, x::AbstractVector, σ²::Real=default_σ²) = FiniteGP(f, x, ScalMat(length(x), σ²))

unfortunately ended up breaking ApproximateGPs, so this reverts that specific change of #236.

I'll move this constructor change into #306 instead.

[codecov]

Codecov Report

Merging #310 (5119b0b) into master (c126765) will decrease coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
- Coverage   97.90%   97.64%   -0.27%     
==========================================
  Files          10       10              
  Lines         382      382              
==========================================
- Hits          374      373       -1     
- Misses          8        9       +1     

Impacted Files	Coverage Δ
src/finite_gp_projection.jl	100.00% <100.00%> (ø)
src/util/common_covmat_ops.jl	95.23% <0.00%> (-2.39%)	⬇️

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