add SuperLU

[Nico Schlömer]

Right now, the docker image is missing the SuperLU solver,

>> list_linear_solver_methods()
Solver method  |  Description                                                 
------------------------------------------------------------------------------
bicgstab       |  Biconjugate gradient stabilized method                      
cg             |  Conjugate gradient method                                   
default        |  default linear solver                                       
gmres          |  Generalized minimal residual method                         
minres         |  Minimal residual method                                     
mumps          |  MUMPS (MUltifrontal Massively Parallel Sparse direct Solver)
petsc          |  PETSc built in LU solver                                    
richardson     |  Richardson method                                           
tfqmr          |  Transpose-free quasi-minimal residual method                
umfpack        |  UMFPACK (Unsymmetric MultiFrontal sparse LU factorization) 

[Prof Garth Wells]

Add SuperLU to PETSc downloads, and remove ML (buggy). Fixes #64.

→ <<cset b02de1a7e3a3>>