Mark table columns as optional as described in the docs

[Hofer-Julian]

Looking at the Ribasim docs the optional table columns are well documented.

These docs don't really reflect what is happening on the Ribasim Python side.

Expectation

If a column is non-optional and it isn't set, Ribasim Python should error with a clear error message.
The information shouldn't be duplicated too often. Ideally it would be shared between docs, Ribasim Core and Ribasim Python, but at least it should be shared between Ribasim Python and Core.

Reality

• Ribasim Python floats and string are nullable and therefore optional if Core allow missing for them
  

  Ribasim/core/src/schema.jl

  Line 68 in cd011cb

  max_flow_rate::Union{Missing, Float64}

• Ribasim Python integers are always optional with default value 0, because node_id needs to have a default because of Ribasim Python implementation details
  

  Ribasim/utils/templates/schemas.py.jinja

  Line 18 in cd011cb

  {{ f[1] }}: {{ f[2] }} = pa.Field(nullable={{ f[3] }}, default=0)

Options

• Switch to the pandas arrow backend which allows null for every data type
• Use pandas nullable integer data type https://pandas.pydata.org/pandas-docs/stable/user_guide/integer_na.html