Fix openPMD: Do not scale unitSI 1

Avoid to scale meshes and particle records with unitSI equal to 1.0. This is often the case for non-floating point records and unintentionally casts their type to double when we mean to stay in integer or unsigned integer (e.g., particle id) scales. This thus fixes issues with the latter and avoids unnecessary operations on large data sets.

cc @ayenpure @berkgeveci

From https://github.com/ECP-WarpX/WarpX/blob/24.07/Examples/Physics_applications/laser_acceleration/inputs_3d : Previously, id and regionofinterest were cast to double, loosing precision for very high numbers.

Backport: release