fem_poisson: take the solve_range as an input, and expect epsilon as a gkyl_array

[manauref]

1. Make fem_poisson take solve_range as an input, rather than constructing it inside. It is better to have the driver program define the ranges in which operations are performed. This was done in fem_poisson_perp and fem_parproj to allow parallel operations; it is strictly not necessary for fem_poisson since this operation always happens on the whole grid, but it keeps this updater in line with the others.
2. Change the signature so that epsilon is specified as a gkyl_array. In the case of constant epsilon, we expect that the gkyl_array contains the DG representation of that constant value. We later do other things inside to make it efficient and not deal with the a whole-grid DG field (in the constant epsilon case).