Edge cases disagree with base R functions

[davidchall]

As raised in #44, boost::math functions throw an exception under certain edge cases (e.g., domain error, pole error). In this scenario, bignum catches the exception and inserts NA in the result vector:

bignum/src/operations.h

Lines 16 to 20 in aeb311c

	try {
	output.data[i] = UnaryOperation(x.data[i]);
	} catch (...) {
	output.is_na[i] = true; // # nocov
	}

In contrast, the base R version of the math function might not return NA. For example, it might return NaN or infinity. We should align the bignum outputs with base R and add these edge cases to the unit tests. I think the correct approach is to pass an error handling policy as an argument to the boost::math function.

Here's a list of known edge cases that cause disagreement between bignum and R:

• digamma(-1)