[geometry] Add basic draft of the Robustness topic for docs

[SVN r86434]

doc/robustness.qbk

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+[/============================================================================
+  Boost.Geometry (aka GGL, Generic Geometry Library)
+
+  Copyright (c) 2013 Mateusz Loskot, London, UK.
+
+  Use, modification and distribution is subject to the Boost Software License,
+  Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+  http://www.boost.org/LICENSE_1_0.txt)
+=============================================================================/]
+
+[/ TODO: this is a basic draft only, should NOT be built into final docs yet ]
+[/ TODO: discuss numerical stability per algorithm (at least for line intersection and point in polygon) ]
+
+[section Robustness]
+
+A numerical stability issues are a common problem in implementations of 
+computational geometry algorithms.
+
+They lead to variety of unexpected sitautions at run-time: an application 
+randomly throws segmentation faults, output computed by an algorithm 
+contains degeneracies, unexpected artefacts or completely invalid.
+
+For example, according to the OpenGIS Simple Feature Specification,
+
+["A Polygon may not have cut lines, spikes or punctures]
+
+From mathematical point of view such condition is easy to verify.
+However, depending on computational method and in the presence of round-off 
+or truncation errors, it is not easy to decided how "sharp" must be a part 
+of polygon in order to consider it a spike.
+
+A 100% robust implementation of an algorithm gives expected result in 100% of cases. Achieving complete floating point robustness implies use of certain set of algorithms as well as platform specific assumptions about floating point representations. 
+
+Despite Boost.Geometry does not promise absolute numerical stability, 
+it attempts to offer balanced efficiency and robustness by:
+
+# selection of algorithms, often solved at case-by-case basis
+# compile-time selection of most precise and capacious C++ type on which to perform computations.
+# support for arbitrary precision numeric types
+
+
+[endsect]
+