Break out plot generation into base plot types and specific implementations

[jamesramsden-bh]

The LBT_TK plots could do with a bit of a reorganisation. Plots should generally be broken down into two levels:

1. Base plot type (e.g. heat map, pie chart, line graph, histogram...) that take in generic types
2. Implementations of the above to fulfil a specific purpose (e.g. a UTCI heat map) that may take in specific types (e.g. hourly continuous collection with a UTCI header)

This has been partially implemented already, e.g. there is a generic heatmap and a UTCI heatmap. However, there are places where specific implementations have not been broken from the base plot type. E.g. a stacked bar chart can only be found inside the utci_comfort_band_comparison method, which requires hourly continuous collecitons of UTCI-headed data.

By breaking out these levels and separating concerns, it makes it possible for a user to create a plot using generic types.

Where possible, try to not change method signatures of existing plots, to help with versioning/back-compatibility.

cc @tg359