unsupportable mixed-precision streaming of particles

As noticed while working on #3405 (closed), a structure with heterogeneous data types (eg, tuple of label,vector) will present problems for reading in contiguous data with a different precision since it may include trailing padding and/or padding between its members.

Currently the only place where this seems to occur are the deserialization of particles. However, these are actually only ever used when streaming between processes (which by definition will have the same precision) and never read from disk. These particle deserializations with precision handling are thus redundant and can/should be replaced with a FatalError if they would somehow be triggered.