Create function for Inventory of Missing Data about cdms.products

[dannyparsons]

Here's the inventory of missing data example plot.

@lilyclements Could you write a function for this? There could be multiple elements and multiple stations. So there could be an option of whether stations or elements go on the x-axis and the other as facets. We could also use fill to have it all on one graph without needing facets, that could be an option too.

We could also have the option of displaying counts or percentage of missing.

Notice that in the image there is the option to "Export Data" so we should have two functions: one to calculate the counts/percentages in tidy format and a graph function which calls the calculate function i.e. both functions have the same input data.

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