MAINT: special: Fix whitespace in cephes/pdtr.c
ENH: special: pdtr and pdtrc give the correct limiting value when m=0.
ENH: special: Return 0 for sufficiently small lambda in pdtrik.
Merge pull request #3155 from WarrenWeckesser/special-poisson-edge-cases

BUG: special: handle the edge case lambda=0 in pdtr, pdtrc and pdtrik