read/readwrite: Add option not to check data size

[ghost]

Some structures (such as SAdsSymbolEntry) are dynamically sized, so the
length cannot be checked with the read data size. Allow the user to
disable length checking on request.

[coveralls]

Pull Request Test Coverage Report for Build 293

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 90.932%

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Files with Coverage Reduction	New Missed Lines	%
pyads/pyads_ex.py	1	86.24%

Totals
Change from base Build 286:	0.1%
Covered Lines:	722
Relevant Lines:	794

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💛 - Coveralls

[stlehmann]

Almost there. Could you please add an entry to the changelog that describes the changes. Then we are ready to merge.