num_pdl statique field should be required

[jmaupetit]

Purpose

For the TIRUERT calculation, we need to sum energy consumption per operator on a determied period. To achieve such task, we need to be able to exhaustively list all PRMs (aka PDLs) attached to operator's stations in a particular day.

Proposal

• the num_pdl field should be required
• as stations related PDLs may evolve in time, we need to perform a daily snapshot of PRMs related to an operator / station
• should the num_pdl field be exported in our open datasets?