Merge pull request #1 from colinbull/develop

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Merge pull request #2 from colinbull/develop

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Merge pull request #3 from colinbull/develop

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Merge pull request #4 from colinbull/develop

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Merge remote-tracking branch 'origin/develop'
Merge remote-tracking branch 'origin/develop'
integer interval functions
Line.empty explicit type parameters
added Line.isContiguousOverInterval
F# 3.1 .NET 4.5.1 upgrade