Adding helper function for each of these with a leading <tdX> tag that
instructs nested_html_table() to inject attributes to the <td> tag.

- Stereo phone plug (a slight modification of a public domain image
  from https://openclipart.org/detail/192396/headphones-connctor ).
- Cable cross-section drawn to match the wire colors in example 08.
- Make the cable jacket and shield colors match the cross-section.
- Images for embedding in the connector and cable nodes are stored
  in a new resources folder.

It specifies how an image will use any extra space available in its
cell. Allowed values are one of these strings:
- FALSE : keep image its natural size. (Default)
- TRUE : scale image uniformly to fit.
- WIDTH : expand image width to fill.
- HEIGHT : expand image height to fill.
- BOTH : expand both image width and height to fill.

The value is sent to Graphviz as a scale attribute to the <img> tag.
Note that there is normally no extra height in the image cell.

It is a list containing minimum width and minimum height of the
image cell. To obtain more available space in the image cell,
this size must be set greater than the natural size of the image.

When True, enclose the image cell in a table without borders to avoid
narrow borders when the fixed width is less than the node width.

@formatc1702 requested changes in his review of wireviz#153:
- Lowercase attribute values
- Nested image attributes
- Avoid sending the whole node object as argument to helper functions
- Simplify html_size_attr()
- Move the resources folder

- Add Optional keyword and value alternatives as requested in review.
- Add a few extra comments about the attributes.

The goal is to enable the user to avoid specifying more attributes
than strictly needed.

- Simplification requested by @formatc1702 in review of wireviz#153
- Inspection of size_html_cell() in Graphviz source code
  https://gitlab.com/graphviz/graphviz/-/blob/master/lib/common/htmltable.c#L1210-1221
  supports that a zero value is treated as not specified.

- Avoid setting image.fixedsize default True when image.scale is
  specified different than "true" or "both" by the user.
- Avoid calculating the missing dimension unless image.fixedsize
  is actually True. It might have been specified False by the user.

- To be able to find an image file with relative path, the directory
  of the .gv file output is injected into each Image data object.
- aspect_ratio() prints a warning and returns 1:1 ratio if it fails.

It is only used during initialization in the __post_init__() function
and does not need to be a proper attribute of the Image dataclass.
https://docs.python.org/3/library/dataclasses.html#init-only-variables

As requested by the owner, it is added to both requirements.txt and
setup.py - see also issue wireviz#172 about a possible redundancy/conflict.