Improve "Polar bar plot" tool

[elijahgreenstein]

Description

1. Rename "Polar plot" as "Polar bar plot'

This PR is in part a follow-up to the discussion of the "Polar plot" processing algorithm in the QGIS Documentation (qgis/QGIS-Documentation#10577). As per the suggestion by @DelazJ , this PR renames "Polar plot" as "Polar bar plot." This is intended to specify the particular kind of Plotly polar chart generated by the processing tool.

2. Improve the "Polar bar plot" tool

The current tool (i) generates a bar for each individual observation and (ii) labels each bar by degree (the bar's position around the plot's origin). For example, here is a plot generated from the capital costs of government projects in Canada:

This PR aims to harmonize the "Polar bar plot" tool with the "Box plot" and "Bar plot" tools by grouping and summing up those values by a designated category. Here is an example of a plot generated with the updated tool, with the capital costs grouped and labeled by Canadian province:

Note that the "category" field (NAME_FIELD) was removed in #64238, because the field did not serve any purpose at that time. This PR restores the NAME_FIELD parameter, and uses that parameter to group the VALUE_FIELD values.

The short description and help text have also been revised to reflect these updates.

3. Make it possible to add a title

This PR follows the example in the Bar plot tool to add a TITLE parameter that users can use to title their plots. (See the same example above.)

4. Remove unnecessary import

The code included a test for a numpy import, but numpy was then left unused in the code. This PR removes the numpy import.