DOC: Improve Doxygen modules documentation

Improve Doxygen modules documentation:
- Add a new `DataReadWrite` group to contain the `WriterObjects` group.
- Use a compact wording for the group name.
- Avoid redundant wording like "This category", "This group", "Filters
  on this category", etc.
- Fix typos, and improve wording.
- Use "ITK" to refer to the toolkit instead of other forms.

Remove references to non-existing classes/objects such as `Pixel`,
`PhysicalImage´, and fix/update the class name for
`BinaryMorphologicalFilterBase` to the current
`itk::BinaryMorphologyImageFilter`.

Take advantage of the commit to adopt a consistent style:
- Use lower cases when referring to technical ITK terms that do not link to
  the class name.
- Use lower cases for Doxygen markup, following the Doxygen documentation.
- Link to ITK classes where possible.
- Finish sentences with a period.
- Indentation.
- Single white spaces between `\defgroup` command, name and title.
- Location of `\sa` commands.
- White lines inside group definition and in between.
- Limit line length to 79+1(CRLF) characters.