Develop path length estimation algorithm

• Look into functions for NGA-East Gulf Coast (GCP) ground motion modification
• Use sediment thickness polygon to compute path lengths
  • Is the effect commutative, so to speak; source-site in-out of GCP or vice versa?
  • Test intersection of source-site line with segments of GCP polygon

@olboyd

mentioned in issue #132 (closed)

Sediment thickness data is sparse and some points inside the coastal plain polygon have zSed = 0. We may need a tighter polygon to use this for path length calculations. The current polygon was drawn by hand with a slight buffer around the zSed data.

What I plan to do to solve this issue is create a grid of zeros and ones, one where sediment thickness is greater than zero and zero where sediment thickness is zero. I will then interpolate this grid along the path from source to receiver and sum up the values, which will then be proportional to the path length through the Coastal Plain.