RLS — Relative/Mobile CRS name is confusing

Although the relative/mobile CRS are defined as 'relative' and
'mobile' in Figure 3, operations are defined as 'static' and 'mobile'
in Figure 4. The names are confusing, and shall reflect the
static/dynamic nature of the coordinate systems.

Introduce new classes (RelativeCRS, RelativeDatum, DynamicRelativeCRS and
DynamicRelativeDatum) to reflect the static/dynamic nature of the relative
coordinate systems separately. RelativeCRS is renamed as StaticRelativeCRS,
and names of related classes are changed in a similar manner. Make MobileCRS
to be derived from DynamicRelativeCRS.
Based on this change, Mobile CRS is a special case of Relative CRS. Therefore,
change the title of section 7.3.1 Relative and Mobile Coordinate Reference
Systems to be 'Relative Coordinate Reference Systems' to avoid confusion.