Ci,Si optimization

[GoogleCodeExporter]

Optimize calculation of sine and cosine integrals, e.g. implement code from
gsl.

Original issue reported on code.google.com by yurkin on 24 Dec 2008 at 7:46

[GoogleCodeExporter]

The expected speedup may be a factor of 10 - see 
http://www.nr.com/forum/showthread.php?t=1412

Original comment by yurkin on 6 Jun 2013 at 8:57

[myurkin]

Another option is discussed in Appendix B of https://doi.org/10.1016/j.ascom.2015.02.002 (thanks to Matthew Lodge for the link).