Refactor code and better error handling in NEST Server

[babsey]

This PR refactors code to better coding structure because I always lost the overview of the old hl_api_server file.

Each file has clear definition:

• hl_api_server: Main file with flask routes and flask configurations (app).
• hl_api_server_helpers: Helper code (objects, functions) for the main file (without flask deps).
• hl_api_server_utils: General code, similar to helpers but without nest imports.
• hl_api_server_mpi: It includes MPI codes.

Plus I added ErrorHandler to use line number of error for NEST Desktop.