Generic speed column for Progress

[NichtJens]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

I was looking for a generic speed column for Progress preferably with customizable units. I only found #926, which hinted at TransferSpeedColumn being a good example, but did not have a resolution.

This PR adds SpeedColumn, which may be used like this:

SpeedColumn() # default units="it/s"
SpeedColumn(units="Hz")

I reuse TaskProgressColumn.render_speed for consistency and extended both TaskProgressColumn.render_speed and TaskProgressColumn itself to also accept custom units.

Regarding the unit tests, I added a line mimicking the test for TransferSpeedColumn as well as extending test_task_progress_column_speed to test TaskProgressColumn.render_speed with custom units. Is there more desired here?

---

I believe, I also fixed a documentation bug in TaskProgressColumn.render_speed, which referred to:

• task (Task): A Task object.

as argument, where it should have been:

• speed (float): Current value of the speed.