Add `HPCConfig` to unify and store `HPCBackend` struct info

[nefrathenrici]

The HPCBackends currently duplicate the same information. They could all wrap one type that stores the common information: experiment_dir, model_interface, module_load_str, hpc_kwargs,
verbose, reruns, exeflags. This will reduce code duplication and improve function signatures.

struct HPCConfig
    experiment_dir::String       = project_dir()
    model_interface::String      = default_model_interface()
    module_load_str::String      = ""
    hpc_kwargs::Dict{Symbol,Any} = Dict{Symbol,Any}()
    verbose::Bool                = false
    reruns::Int                  = 1
    exeflags::String             = ""
end

struct CaltechHPCBackend <: SlurmBackend
    config::HPCConfig = HPCConfig()
end