Introduce the GaussianProcessProbabilisticProgramme

[willtebbutt]

TODO:

• [x] Deprecate model macro
• [x] Write docstrings + doctests
• [x] Update readme with new example
• [x] Remove conditioning and approximate conditioning functionality (we get it from AbstractGPs)
• [x] Remove all remaining redundant code

[codecov[bot]]

Codecov Report

Merging #161 (95f6802) into DEV-0.7 (7e3ab54) will decrease coverage by 4.01%. The diff coverage is 94.44%.

@@             Coverage Diff             @@
##           DEV-0.7     #161      +/-   ##
===========================================
- Coverage    82.78%   78.77%   -4.02%     
===========================================
  Files           21       18       -3     
  Lines          552      457      -95     
===========================================
- Hits           457      360      -97     
- Misses          95       97       +2     

Impacted Files	Coverage Δ
src/Stheno.jl	100.00% <ø> (ø)
src/abstract_gp.jl	75.00% <ø> (-16.67%)	:arrow_down:
src/composite/addition.jl	100.00% <ø> (ø)
src/composite/compose.jl	68.18% <ø> (ø)
src/composite/cross.jl	97.36% <ø> (-1.00%)	:arrow_down:
src/gaussian_process_probabilistic_programme.jl	94.33% <94.33%> (ø)
src/util/abstract_data_set.jl	100.00% <100.00%> (ø)

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