[IMP-dev] Several small patches

[Daniel Russel <>]

First I added a pair of functions internal::assert_failure and  
internal::check_failure which get called when assertions and checks  
fail. You can have the debugger break in these to find failures.

Attachment: error_func
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The derivative accumulator checks for NaN.

Attachment: da_nan
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And finally ConjugateGradients wasn't initializing its threshold_  
member variable.

Attachment: cg_init
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