Create Glacier or Gdir type to store glacier state and initial conditions

[JordiBolibar]

Same as for the case of the Simulation type. We have too many basic types going around, and we need to tidy things up. Instead of passing so many matrices and scalars to the solver, we should gather everything in a type Glacier to be passed to simulations.

We have two options here:

• Investigate if we can directly use the gdirs, and if this is technically feasible both with OrdinaryDiffEq.jl solvers and with AD in UDEs. The fact that it will be a PyObject might make things complicated.
• Otherwise we could just create a Gdir type in Julia to translate it, or just call it Glacier. We should also make sure that types such as mutable structs or Dicts work correctly with solvers and AD. To be thought.

[JordiBolibar]

This could potentially be implemented with a custom type based on MeshArrays.jl: https://github.com/JuliaClimate/MeshArrays.jl

[JordiBolibar]

Closed as it's mainly followed in #82