Add special named flag for invalid particles (AMReX-Codes#3688)

This lets you do 

`p.m_idcpu = LongParticleIDs::InvalidParticleID;` 

instead of 

`amrex::ParticleIDWrapper{p.m_idcpu} = -1;`

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [x] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

---------

Co-authored-by: Axel Huebl <[email protected]>

Src/Particle/AMReX_Particle.H

@@ -15,7 +15,7 @@ namespace amrex {
 
 namespace
 {
- /** Used for 64bit Long particle Ids as in AoS layout */
+ /** Special flags used for 64-bit Long particle Ids */
  namespace LongParticleIds {
  constexpr Long GhostParticleID = 549755813887L; // 2**39-1
  constexpr Long VirtualParticleID = GhostParticleID - 1;
@@ -25,6 +25,15 @@ namespace
  }
 
  using namespace LongParticleIds;
+
+ /** Flags used to set the entire uint64_t idcpu
+ to special values at once.
+ */
+ namespace ParticleIdCpus {
+ constexpr std::uint64_t Invalid = 16777216; // corresponds to id = -1, cpu = 0
+ }
+
+ using namespace ParticleIdCpus;
 }
 
 struct ParticleIDWrapper