Also test OpenMP with 3 and 4 threads (currently failing)
For now, evaluate the threshold and SpikeGeneratorGroup in a single thread. The previous solution assumed knowledge about the static scheduling that I don't think we have. Fixes #358
Merge pull request #359 from brian-team/fix_358

Fix issuse #358
Refer to Brian2 as Beta, not as Alpha
Add release notes for Brian 2.0beta
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Add "Problems with numerical integration" to the known issues
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Document linked variable mechanism
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Added warnings about potential problems with OpenMP code
Small doc fixes
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Final update of the number of bugs in the release notes
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