Specify that the computation of checksums includes counts.

[DennisHeimbigner]

re: Issue #33
re: Discussion #6
re: PR #34

Modify the checksum section to add a specific subsection on how the checksum is computed. This specifies that the computation specifically include the counts for variable-length data (Strings, Sequences, and Structs with variable-length fields).

[Mikejmnez]

I agree that it is important to be more explicit about how Sequences / Structures are handled when computing checksums. I found the following in the original text:

dap4-specification/01_data-model-and-serialized-rep.adoc

Lines 1737 to 1740 in 406ab93

	. The Sequence objects are treated '`like strings`' and prefixed with a
	length count. In the last of the three variables, the dimensioned
	sequence _x-star_ has two sequence instances where the first sequence
	has 3 elements and the second has 6.

The current text does not mention Structures or Groups.

[DennisHeimbigner]

I think that text is referring to the chunked format counts as opposed to the internal sequence and string counts.