Allow AbstractGPs in WrappedGP

[willtebbutt]

This is a bug fix.

Currently, WrappedGPs only permit GPs to be wrapped, which is limiting.

An unrelated change is the removal of a lot of block-diagonal code that was redundant and broken.

[codecov]

Codecov Report

Merging #217 (270188d) into master (999cd2c) will increase coverage by 3.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   86.42%   89.53%   +3.10%     
==========================================
  Files          17       16       -1     
  Lines         442      344      -98     
==========================================
- Hits          382      308      -74     
+ Misses         60       36      -24     

Impacted Files	Coverage Δ
src/gp/gp.jl	100.00% <100.00%> (ø)
src/util/proper_type_piracy.jl	0.00% <0.00%> (-100.00%)	⬇️
src/util/block_arrays/dense.jl	70.58% <0.00%> (-29.42%)	⬇️
src/composite/composite_gp.jl	100.00% <0.00%> (ø)
src/abstract_gp.jl	87.50% <0.00%> (+1.78%)	⬆️

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