rename particle "tag" to "id"
The property "tag" in mdsim.particles should be renamed to "id". An "id" is a unique identifier, while a tag can be anything. This change corresponds to the naming convention in the H5MD format:
Note that the change breaks the Lua API of mdsim.particle:
It is tempting to replace the functions get_tag() and set_tag(x) by a read-write property "tag" (and similarly for all other properties). Before, we have to check that this does not break anything else, e.g., because get_tag is used as a slot function for output.