Refactor code branches based on meshBody.hasParticles()

After the merging of PR https://github.com/GEOS-DEV/GEOS/pull/1756/, there will be a few locations in the code where we will have unsightly logical branches depending on whether a meshBody is discretized with particles or not. These should be streamlined by or immediately after the meshLevel->meshDiscretization PR.

These are the specific functions as flagged by the comments below. They can also be easily found by searching for hasParticles().

1. ProblemManager::generateMesh: "mesh bodies with particles load their data into particle blocks, not cell blocks"
2. ProblemManager::generateMesh: "particle mesh bodies don't have a finite element discretization"
3. ProblemManager::setRegionQuadrature: "branch due to difference in particle vs cell regions"
4. DomainPartition::setupCommunications: "Currently, particle-based mesh bodies do not construct their own domain decomposition"
5. MeshManager::importFields: "field import is not currently compatible with particle mesh bodies"