Adding plot_kernel() function to utils

[AndrewILWilliams]

Example usage below. Hoping this will make exploring different kernels a bit easier for users.

To-do:

• [] Add tests

[codecov]

Codecov Report

Merging #13 (04a4093) into master (95613d2) will decrease coverage by 0.21%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
- Coverage   97.03%   96.81%   -0.22%     
==========================================
  Files           7        7              
  Lines         438      440       +2     
==========================================
+ Hits          425      426       +1     
- Misses         13       14       +1     

Impacted Files	Coverage Δ
GCEm/__init__.py	96.42% <80.00%> (-1.14%)	⬇️

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[duncanwp]

Nice, thanks! I recently factored the kernel creation in to a separate function (_get_gpflow_kernel), could you use that instead of replicating it here?

I'm a little uncomfortable with the lack of test coverage in utils.py too. Testing plots is a pain but maybe we should split the plotting routines in to a separate file?

[AndrewILWilliams]

Nice idea! I'll look into that now

In terms of tests I was thinking primarily of checking that the assertions fail correctly (if the user gives an incorrect kernel name) and maybe also checking that the axes output has the right shape? Currently I've set it up so it gives an adaptive number of subplots depending on the input kernel list and whether you also want a "Product"/"Sum" plot as well

[AndrewILWilliams]

@duncanwp I've made a small change to _get_gpflow_kernel so that it can also return the list of individual kernels if asked to. Now calling this in the kernel_plot() function rather than reproducing that code

[duncanwp]

I like the idea but I'm not a big fan of optionally returning a different number of arguments, it can make code a bit hard to visually debug. Looking at it again maybe get_kernel should just initialise a single kernel, and there should be a separate (simple) function for combining them?