kernel should allow degree limits

[mikestillman]

trafficstars

If we know that the kernel of a ring map is generated in degree d, then we should be able to tell M2 that info. This issue is from the ask mike session at the M2 day at Fields, Oct 21, 2016.

R = ZZ/32003[a..d]
S = ZZ/32003[s,t]
phi = map(S,R,{s^4, s^3*t, s*t^3, t^4})
ker(phi, DegreeLimit=>3)