Add Custom+Partial Iteration for Particle Scatter and Gather Operations

[aliemen]

This pull request implements the following changes to resolve issue #326:

1. Scatter Operation Enhancements:

   • Modified ParticleAttrib::scatter to accept a custom range policy and hashing array, like ippl::hash_type.
   • Added a check to ensure the custom policy matches the number of available elements.
   • Introduced the usage of mapped_idx instead of idx for particle scattering.
   • Overloaded the non-class scatter interface to preserve old functionality while allowing custom range policies.

2. Gather:

   • Modified ParticleAttrib::gather to include a boolean parameter that determines whether to add to or overwrite the attribute's values.
   • Changed the non-class gather interface to maintain backward compatibility.

Testing:

• Added a new test file test/particle/TestHashedScatter.cpp.
• Modified the existing TestGather to validate the new functionality (use "+=" instead of "+").

closes #326