Restore cmap to config; simpler config of cmap/dpi/image format

Closes #776

+ Aligns command line and configuration field names with those in matplotlibrc files.
+ Restores the `cmap` configuration option to `default_config.yaml` and introduces `savefig_dpi` option.
+ Adds command line options for setting DPI (`--savefig-dpi`), Colormap (`--cmap`)and Output file
  format (`--savefig-format`).
+ Expands documentation on how to use custom configuration files or command line options to set the DPI/Colormap/Output
  format.
+ Updates the header to `topostats.mplstyle` to explain how to use it as typically users will have created a copy of the
  file (after the convenience function `topostats create-matplotlibrc` was introduced with #773).
+ To achieve this the dictionary `config["plotting"]` needed explicitly updating as the `update_config()` function
  doesn't update nested configurations (since this is the first PR that introduces command line options that modify any
  of the values in the nested dictionaries).
+ Updates options for `topostats toposum`` to align with `savefig_format` and adds flag to entry point so output format
  is consistent.
+ Updates and expands the configuration documentation explaining how to use these conveniences.

As a consequence quite a few files are touched to ensure that validation and processing functions all have variables
that align with those in the configuration.

If users could test this it would be very much appreciated, if you use the Git installed version something like the
following would switch branches and allow you test it.

```
conda create --name topostats-config # Create and activate a virtual env specific to this
conda activate topostats-config
cd ~/path/to/TopoStats
git pull
git checkout ns-rse/776-config-jigging
pip install -e .
topostats process --output-dir base
topostats create-config test_config.yaml   # Create test_config.yaml to try changing parameters
topostats process --config test_config.yaml --output-dir test1
topostats process  --output-dir test2 --savefig-dpi 10 --cmap rainbow --savefig-format svg
topostats process --config test_config.yaml  --output-dir test3 --savefig-dpi 80 --cmap viridis --savefig-format pdf
```

Each invocation of `topostats process` will save output to its own directory (either `base`, `test1`, `test2` and
`test3`) for comparison. There should be differences between each `base` the values used in `test_config.yaml` and
saved under `test1` and those under `test2` and `test3` should also differ.

I would really appreciate feedback on the documentation as without clear documentation it is perhaps confusing how the
components interact and work and can be modified and getting this as clear as possible will be really helpful.

topostats/default_config.yaml

@@ -62,7 +62,7 @@ plotting:
   run: true # Options : true, false
   style: topostats.mplstyle # Options : topostats.mplstyle or path to a matplotlibrc params file
   savefig_format: null # Options : null (defaults to png) or see https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.savefig.html
-  savefig_dpi: null # Options : null (defaults to format) see https://afm-spm.github.io/TopoStats/main/configuration.html#further-customisation and https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.savefig.html
+  savefig_dpi: null # Options : null (defaults to figure) see https://afm-spm.github.io/TopoStats/main/configuration.html#further-customisation and https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.savefig.html
   pixel_interpolation: null # Options : https://matplotlib.org/stable/gallery/images_contours_and_fields/interpolation_methods.html
   image_set: core # Options : all, core
   zrange: [null, null] # low and high height range for core images (can take [null, null]). low <= high