Create toplevel io.rr tree structure for RainfallRunoffModel input

[arthurvd]

Current tree structure under hydrolib.core.io is now a mixture of many FM (sub)files, DIMR, and a few RR (sub)files.
Let's improve it as follows:

• hydrolib/core/io/
  • rr/: all RR_related files
    • models.py: with the RainfallRunoffModel and its toplevel components (currently none), and references to child models.
      Note: this means that the 'fnm/' dir will disappear.
    • meteo/: rainfall and other meteo files
      • here the contents of the current 'io/bui/' dir.
    • topology/: Topology layer (node + link file, see D.19.13 in https://content.oss.deltares.nl/delft3d/manuals/SOBEK_User_Manual.pdf#subsection.D.19.13 )
      This was now called 'rr/network/' in branch feature/138_add_rr_network_topology_node_file
      • models.py: with the Node and Link models together.
    • in the future, more subdirs under 'rr/' may appear, for example 'boundary/', 'paved', etc. (See subsections D.19 in SOBEK UM)
      Note: this part of the work under other future issue.
  • fm/: all FM related files
    Note: this part of the work under other future issue/
  • dimr/ all DIMR related files
    Note: this part of the work under other future issue/

In addition to this directory reorganization, also some additional work in this same issue:

• hook up the current Node and Link model classes into the RainfallRunoffModel.node_data and .branch_data attributes.
• Add a root validator to RainfallRunoffModel that checks for all branch data links whether their start and end node id is present in the node_data nodes.