Add gauge_equiv options

[kvid]

Avoid gauge equivalents bugs as reported in #282 by enabling the user to select the proper method for computing equivalents that fits the current wire harness.

1. The current implementation requires Python 3.10+. Rewrote slightly to support older versions. or change the Wireviz required Python versions.

2. Find proper default values for the new options. Please test this branch and suggest default values that might be useful for most users.

Alternative 1:

options:
  gauge_equiv:
    show: false
    rounding: nearest
    mm2: 3  # digits precision
    awg: ['0000', '000', '00', 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40]
    match:
      poor: 10
      invalid: 50

Alternative 2 - same as alternative 1 above, except listing mm² values available:

    mm2: ['0.09', '0.14', '0.25', '0.34', '0.5', '0.75', '1', '1.5', '2.5', '4', '6', '10', '16', '25', '35', '50']

These values are just the keys of the old awg_equiv_table. Can someone suggest a more complete set of standard values?

3. Should probably rename Cable.show_equiv to Cable.gauge_equiv for consistency with the new options attribute and to improve code readability. In addition, it removes a conflict with a possible future length_equiv attribute.

4. Maybe consider also supporting kcmil?

Note that the initial commit 8dc1219 is not part of this PR, but has its own PR #498 that should be merged before this one.

[sbennett18]

Minor typo precentage --> percentage

[kvid]

Thank you for reviewing and catching my typo!

I didn't expect much attention about this PR while it still is a draft, so your message was a nice surprise. 😄