Modular Abelian Varieties

Todo on March 16

(william) This is completely wrong (the modabvar function on modular symbols assumes it's ambient!):

sage: m = ModularSymbols(37)[1]
sage: m.modular_abelian_variety()
Jacobian of the modular curve associated to the congruence subgroup Gamma0(37)

(william) Move functions out of abvar_modsym_factor into abvar and delete that file.
(william) Torsion subgroups:
- Refactor base class
- Get implementation to work with defining data being (lattice, abvar); compute generators.
- Quotients by finite subgroup
(william) Decomposition:
- three types:
  - ungrouped as simple abvars (default)
  - groups abvars
  - over End(A)
  - deprecate hecke_decomposition
- label function
- create from label
(craig) Compute the Hecke algebra image in End(A) and find a good clean way to represent for Hecke stable. New object that is a subring of End(A). Have methods like R.index_in(S).
(craig) Compute End(A):
- for simple Af
- in general.
(craig) Degeneracy maps
(william/craig) Morphisms:
- Kernels
- Cokernels
(william/craig) Isogenies:
- Complementary -- invert matrix, clear denom.
- Dual
(william) Intersection pairing
(?) Poincare Reducibility:
- projection
- quotients by abelian subvariety
(craig/william) Minimal isogeny degree for A, B simple.
(craig/william) Create a small mock database