[Maxima] context based version of abs_integrate--no more simp_assuming

[Barton Willis]

I committed a new version of abs_integrate that uses contexts instead of the buggy simp_assuming macro.  The new code doesn't have a general purpose 
with_assumptions function (or macro) replacement for simp_assuming.

The new version also fixes some other bugs (rtest_abs_integrate bugs 148 and 149).

--bw