Support for Nonuniform grid spec in grid_size

[tylerflex]

Still work in progress.

The components of grid_size now accept lists of non-negative floats in addition to non-negative floats.

if list, the simulation.grid creation is augmented to include a boundary coordinate generation based on user specification of nonuniform mesh via grid_sizes.

The rules governing this generation are as specified as follows for a list of grid sizes along an axis (call this A).

• The coordinates are placed onto the simulation center and size specifications such that the midpoint of A (in terms of sum()) is located at simulation.center[axis].
• If the sum() of A does not fully cover the simulation.size[axis], the first and last values of A are repeatedly added to the coords until filling up the domain.
• The simulation size (and corresponding coordinates) is snapped down to include only coordinates that lie within (inclusive) the simulation center and size bounds.
• If PML layers are included, the extreme (min and max) values of the snapped A are used to tile these layers on the beginning and end of the simulation.

Please comment if you have a clarification or issues with this definition and see test to get a feeling for the expected output given my implementation in this PR.

[tylerflex]

fixes #45

[momchil-flex]

Looks good.