I have noticed recently that when evaluating the EI function at any
number over 20, it adds 2pi i, which shouldn't be there. After looking
at the code it appears that it uses scipy, so the error is probably
there. However, when looking for alternatives, I found the
exponential_integral_1 from PARI function which doesn't seem to suffer
from the same issue as scipy. When I did a test using the identity Ei
(x) = -exponential_integral_1(-x), PARI's implementation was about
twice as fast as scipy and had a much greater precision.
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