Weak formulation of Poisson equation
Model non-constant viscosity with fem::cell_quan
Link with tcmalloc
Replace ublas_matrix with vector of maps
Revert "Ditch uBlas matrix, replace it with vector of maps."

This reverts commit 34f88264f91eca8d387fee5e174beb8ca4fffa05.
Merge branch 'weak_form'