Adapted complex exp() function so that it can handle inf and nan arguments as well

[JorisDeRidder]

Why this PR?

The current version of the complex exp() function is not able to handle arguments that contain +/- inf or NaN in their real or imaginary part. This impacts other complex functions that use the exp() function.

For example, for the most widely used implementation of the complex Faddeeva function w(), the current exp() implementation leads to w(1e160 - 1e159*i) = NaN + NaN *i , while the correct value is -5.586035480670854e-162 + 5.5860354806708545e-161 * i. The underlying reason is that the current exp() implementation erroneously returns exp(-inf + inf *i) = NaN + Nan *i instead of the correct 0 + 0*i.

Cf also issue #103.

Contents of this PR

• I propose a modified complex exp() function that does deal with inf and nan. The added logic was strongly inspired by the one implemented in the <complex> C++ header that comes with clang++.
• I added extra unit tests. The relevant values were taken from this page.
• For the unit tests I also implemented close_naninf() and close_naninf_to_tol() as the existing functions close() and close_to_tol() are not able to deal with inf and nan.

[cuviper]

• I added extra unit tests. The relevant values were taken from this page.

Could you please add that reference link as a comment in the test?

[JorisDeRidder]

• I added extra unit tests. The relevant values were taken from this page.

Could you please add that reference link as a comment in the test?

OK. Done.

[JorisDeRidder]

A gentle reminder...

[cuviper]

bors r+

[bors[bot]]

Build succeeded:

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