sPuReMD: refactor contiguous and custom charge constraint code. Make applicable API functions available to all interfaces.

sPuReMD: fix issue with API functions (setup2 / reset2 -> cleanup) regarding new contiguous and custom charge constraint implementation (identified and suggested fix by Cagri Kaymak).

sPuReMD: do not compute hydrogen bond interactions when there are no valid force field parameters available for the given triplet of atom types. Allow arbitrary numbers of H-bond interactions per atom.

sPuReMD: fix issue with sparse charge matrix (for preconditioning) not being reallocated upon out-of-memory condition. Fix issues where charge matrix and bond/H-bond list entries were reading from and potentially writing to invalid memory locations. Remove unused code for grid. Other code clean-up.

sPuReMD: fix uninitialized variable issue with API functions (charge constraints => setup2, reset2).

sPuReMD: add error checks around I/O routines to silence warnings regarding disregarded return values.

Fix memory allocation issues related to API usage (setup => cleanup but no simulate) as detailed in Issue #9.

sPuReMD: rework memory management logic to better match PG-PuReMD code. Adapt new memory management logic for multiple simulation logic (as motivated by memory issues observed with QM/MM code for AMBER integration work). Other code clean-up.

sPuReMD: change dummy atom behavior to participate only in Coulomb interactions (exclude from van der Waal interactions in this changeset).

sPuReMD, PG-PuReMD: add control file keyword (include_polarization_energy) to toggle polarization energy calcuation in ReaxFF.

sPuReMD: rework charge solver for QMMM to not require explicit masking (MM atom fixed charges are not respected). Other code clean-up.

sPuReMD: ensure reallocation for multi-simulation runs happens correctly for the charge solver codepath.

sPuReMD: ensure matrix reallocation correctly occurs in multiple simulation cases where the linear system size increases.

sPuReMD: add dummy atom support for other geometry file formats (PDB, custom). Rework interal logging during debugging for memory management. Other code clean-up.

sPuReMD: update QMMM interface for specifying molecular charge constraints for charge model (EEM only currently).

sPuReMD: fix issue with atoms exactly on the upper boundary of the simulation box not being correctly binned into grid cells for the cell method. Refine conversion constants for kinetic energy and temperature calculations to obtain better agreement with the Fortran ReaxFF codebase.

sPuReMD: fix code to remove several compiler warnings. Remove unused code. Disable compilation of several debug routines. Other code clean-up.

sPuReMD: add support for user-defined molecular charge constraints with EEM (using MOLCHARGE keyword in BGF geometry files). Fix issue with far neighbor list not being initialized for certain edge cases.

sPuReMD: enable setting the number of OpenMP threads via control file keyword or OpenMP environment variable (OMP_NUM_THREADS).