Update README.md

Added BMI description and link to BMI article in Computers & Geosciences.
Removed CMI constants.
Added BMI_POISSON namespace, removed unused funcs.
Moved model-specific bmi.h to bmi_poisson.h.
Header file for just BMI_ namespace.
Moved Poisson-specific code into their own files.
Added a main for poisson model.
Renamed bmi_impl.c to bmi_poisson.c.
Added poisson_free function.
Added BMI_POISSON namespace to prototypes.
Use functions from poisson.c.
Rename get_grid_nbytes and grid_grid_size funcs.

Changed
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* Renamed:
  - BMI_POISSON_Get_grid_nbytes -> BMI_POISSON_Get_var_nbytes
  - BMI_POISSON_Get_grid_size -> BMI_POISSON_Get_var_size
  since they have to do with the values not the grid itself.
Rename to poisson.pc.cmake.
Update to new library name, bmi_poisson.