Make `Union{Nothing, Matrix{F}}` struct fields parametric

[JordiBolibar]

In order to avoid issues with mixed types, especially for Enzyme, we should stick to Matrix{F} types for anything related to Glacier2D, SIA2D, etc... This will ensure that the type doesn't change.

The types should be made parametric, i.e. moving them to the function signature instead of individually specifying them in each field.

To be extended to Huginn.jl, Muninn.jl and ODINN.jl.

[facusapienza21]

@JordiBolibar using Array is not the same? Vector and Matrix are just aliased for 1 and 2 dimensional Arrays.

[JordiBolibar]

We should double check if this is still a problem. I no longer remember why this was a problem for Enzyme. It might be linked to the Nothing part of the Union, and not the Matrix{F}.