Example scripts using third angle projection added

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Method created to add single view to drawing sheet, avoiding code duplication as proposed in issue #12.

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Serious and previously undiscovered bug fixed: in previous versions, the positions on the page of views from the positive and negative x directions were swapped from the correct BS 8888:2011 first-angle projection positions
Additional example invocation scripts for testing the module inserted (the above-mentioned bug went undetected for so long because all of the examples in the "test suite" had reflection symmetry about xz and yz planes, and hence were unaffected by the bug, so in particular, some less symmetric examples are now included)
Module will now use the TechDraw workbench instead of the Drawing workbench, _if_ the FreeCAD version number under which the module is run is 0.19 or later (Drawing toolbox still used when module is run under earlier versions of FreeCAD).
Previously-documented "wastage of RAM due to dummy documents left open" bug fixed, _if_ the FreeCAD version number under which the module is run is 0.18.4 or later (behaviour in this respect unchanged when module is run under earlier versions of FreeCAD).

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Documentation and code comments updated with new debugging test results

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