native floating point sinint

[Colin Macdonald]

Hi Joel,

I wonder if you could take a look at this old pull request:

https://github.com/cbm755/octsympy/pull/425

My feeling is drop this but I thought you might offer a perspective 
after your summer project.

Background: the symbolic package inherited a bunch of code from the 
specfun pkg.  This included "sinint" and "cosint" implementations.  They 
were not very good so I replaced them [1] with rather slow, 
nonvectorized calls to the SymPy (ultimately the arbitrary precision 
python library "mpmath").  The PR is the original specfun code, with a 
few failing tests added by me.

[1] https://github.com/cbm755/octsympy/blob/master/inst/%40double/sinint.m