add tolerance documentation

based on the summary by @giuseppeverduciALMA (https://github.com/buildingSMART/IDS/blob/0d50fd8f2dbd5b388f6fafb67da255cc3ce2b4ca/Documentation/tolerance.md). #315

Documentation/tolerance.md

@@ -0,0 +1,61 @@
+# Tolerance value in IDS
+
+## Equality tolerance
+
+Because of rounding errors, a tolerance value must always be considered for the equality of floating-point numbers (doubles in ids:simpleValue and xs:restriction). 
+
+To support both low numbers, like an area of a wire expressed in square meters, and high numbers, like a length of a railway, the IDS uses both a relative and a fixed component, following the formula:
+
+`x == v ⇒ (v - abs(v) × ϵ - ϵ) < x < (v + abs(v) × ϵ + ϵ)` 
+with a tolerance value being: `ϵ = 1.0e⁻⁶`
+
+The table below demonstrates characteristic ranges of values within the tolerance:
+
+|v|lower bound<br>`(v - abs(v) × ε - ε)`|	upper bound<br>`(v + abs(v) × ε + ε)`|	absolute delta|
+|---:|-------------:|-------------:|--:|
+| 100000.0 |		99999.899999 |	100000.100001 |	0.200002 |
+| 10000.0	 |	9999.989999	| 10000.010001 |	0.020002 |
+| 1000.0	 |	999.998999	| 1000.001001 |	0.002002 |
+| 100.0	   |	99.999899	| 100.000101 |	0.000202 |
+| 10.0     |		9.999989	| 10.000011 |	2.2E-05 |
+| 1.0	|	0.999998	| 1.000002 |	4E-06 |
+| 0.1	|	0.0999989	| 0.1000011 |	2.2E-06 |
+| 0.01 |		0.00999899 |	0.01000101 |	2.02E-06 |
+| 0.001	|	0.000998999	| 0.001001001 |	2.002E-06 |
+| 0.0001	|	0.0000989999 |	0.0001010001 |	2.0002E-06 |
+| 0.00001	|	0.00000899999 |	0.00001100001 |	2.00002E-06 |
+| 0.000001	|	-0.000000000001 |	0.000002000001 |	2.000002E-06 |
+| 0.0000001	|	-0.0000009000001 |	0.0000011000001 |	2.0000002E-06 |
+| 0	|	-0.000001 |	0.000001 |	2.0E-06 |
+| -0.0000001	|	-0.0000011000001 |	0.0000009000001 |	2.0000002E-06 |
+| -0.000001	|	-0.0000020000 |	0.000000000001 |	2.000002E-06 |
+| -0.00001	|	-0.0000110000 |	-0.00000899999 |	2.00002E-06 |
+| -0.0001	|	-0.0001010001 |	-0.0000989999 |	2.0002E-06 |
+| -0.001	|	-0.0010010010 |	-0.000998999 |	2.002E-06 |
+| -0.01	|	-0.0100010100 |	-0.00999899 |	2.02E-06 |
+| -0.1	|	-0.1000011000 |	-0.0999989 |	2.2E-06 |
+| -1.0	|	-1.0000020000 |	-0.999998 |	4E-06 |
+| -10.0	|	-10.0000110000 |	-9.999989 |	2.2E-05 |
+| -100.0	|	-100.0001010000 |	-99.999899 |	0.000202 |
+| -1000.0	|	-1000.0010010000 |	-999.998999 |	0.002002 |
+| -10000.0	|	-10000.0100010000 |	-9999.989999 |	0.020002 |
+| -100000.0	|	-100000.1000010000 |	-99999.899999 |	0.200002 |
+| -1000000.0	|	-1000001.0000010000 * |	-999998.999999 |	2.000002 |
+
+\* If the least significant digits are beyond the precision of a IEEE74 double (approximately 17 digits), it might result in a false positive. For example, -1000001.00000100001 would still pass as being equal to -1000000.0, even though being below the lower bound.
+
+Note: the tolerance value is not configurable, as it is specific only for rounding errors, not for construction-related tolerances, for which users need to provide an explicit range.
+
+## Range tolerance
+
+Adding tolerance to range checks would open up all kinds of strange edge cases like 41.999958 being both in the range 42-50 inclusive, but also  < 42 exclusive. For that reason, **exclusive ranges should be shrunk, while inclusive ranges should be stretched**.
+
+For example:
+- `x > 0 ⇒ x > 1.0e⁻⁶` 
+- `x >= 0 ⇒ x >= -1.0e⁻⁶`
+
+If a numeric range constraint's spread is lower than the agreed limit, the Audit-tool should raise a warning.
+
+# 
+
+Note: the history of these agreements can be traced in the history of [issue 78](https://github.com/buildingSMART/IDS/issues/78), [issue 36](https://github.com/buildingSMART/IDS/issues/36), and in [the summary by @giuseppeverduciALMA](https://github.com/buildingSMART/IDS/blob/0d50fd8f2dbd5b388f6fafb67da255cc3ce2b4ca/Documentation/tolerance.md).