Improve docs and tests for NcML logical view elements

[cwardgar]

From #778.

• logicalSection, logicalSlice, and logicalReduce are only briefly described in AnnotatedSchema4.adoc. We should expand on this.
• Only logicalReduce has an example in AnnotatedSchema4.adoc. All 3 elements should. Ideally, those examples would be drawn from unit tests, so that they're less likely to become stale.
• The variable Element section doesn't include logicalReduce.
• Cookbook.adoc includes an example for logicalReduce (the exact same one as in AnnotatedSchema4.adoc), but not the other 2 elements.
• Only logicalReduce is tested, in TestLogicalReduce.

[cwardgar]

Here is one example of logicalSection. I didn't realize before that the logical view stuff are child elements of variable, not attributes of it.

[cofinoa]

@cwardgar the most tricky part of logicalSections and slices are the renaming of the anonymous dimensions created when the sections are extracted.

To do the renaming a wrapping with aggregation and netcdf elements are required making quite complex ncml.

The other problem it's about the logical of the ncml parsing it's been made by the CDM library which it's dependent on the order of the elements are been parsed and it's not based on the order they are written.

[cwardgar]

#998 addresses the last item by replacing the crappy TestLogicalReduce with LogicalViewsSpec.