Use of Integer instead of Int

Currently, several core data structures in `Ferrite` use `Int` explicitly for numerical types. For example:

```julia
struct Line <: AbstractCell{RefLine}
nodes::NTuple{2, Int}
end

struct QuadraticLine <: AbstractCell{RefLine}
nodes::NTuple{3, Int}
end

mutable struct SubDofHandler{DH} <: AbstractDofHandler
const dh::DH
const cellset::OrderedSet{Int}
const field_names::Vector{Symbol}
const field_interpolations::Vector{Interpolation}
const field_n_components::Vector{Int}
ndofs_per_cell::Int
end

mutable struct DofHandler{dim, G <: AbstractGrid{dim}} <: AbstractDofHandler
const subdofhandlers::Vector{SubDofHandler{DofHandler{dim, G}}}
const field_names::Vector{Symbol}
const cell_dofs::Vector{Int}
const cell_dofs_offset::Vector{Int}
const cell_to_subdofhandler::Vector{Int}
closed::Bool
const grid::G
ndofs::Int
end
```

The issue is for other code that uses Ferrite and wants to use (e.g. `Int32` or `Int16`), it's a bit cumbersome, and it's impossible in Cell types.