Tidy up and deprecate approx_posterior

[willtebbutt]

No description provided.

[codecov]

Codecov Report

Merging #218 (76ab951) into master (c822896) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #218   +/-   ##
=======================================
  Coverage   89.53%   89.53%           
=======================================
  Files          16       16           
  Lines         344      344           
=======================================
  Hits          308      308           
  Misses         36       36           

Impacted Files	Coverage Δ
src/sparse_finite_gp.jl	93.33% <ø> (ø)
src/util/dense.jl	70.58% <ø> (ø)

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