Fixed include issues.

Fixed
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* Changed to not include bmi.h or sedflux_api.h headers, which had
  become obsolete. Instead, use the same template code that is at the
  start of bmi.h (bmi_sedflux2d.h) to define BMI stuff. This should
  really be combined into a single file and shared between sedflux2d
  and sedflux3d headers.