Add B20 GMM tables

Add three B20 GMMs - AB14mod1, AB14mod2, and BCA10D. The Boore (2020) site amplification model is not included in this MR.

See #137 (closed)

Table extrapolation from published B20 data files:

• tables were generated from the vs30 = 3000 m/s reference site files
• B20 revises B18 models with the same name by applying a factors:
  • convert geometric mean of two horizontal components to RotD50 (column RotD50divGM_AR)
  • path adjustment (column antiP)
  • offset adjustment (column aniO)

Extrapolation

published B20 tables include GMs for 2 km <= Rrup <= 1200 km and 3.0 <= Mw <= 8.0, so tables must be extrapolated down to 0 km, up to 1500 km, and up to M 8.2. The GMs are taken from column gmimdk80gGM2RotPO, "the final adjusted B18 GMIM" for RotD50.

• GMs for M8.2 are set equal to published GMs for M8.0
• GMs for Rrup 1500 km are set equal to Rrup 1200 km
• Though no value for gmimdk80gGM2RotPO is provided for 0 and 2 km, distance-specific factors RotD50divGM_AR, antiP, and antiO are provided, so GMs for 0 and 1 km are set equal to the unmodified B17 GM (column gmimdk80g) multiplied by these factors:
  • gmimdk80gGM2RotPO [1 km] = gmimdk80g [2 km] x RotD50divGM_AR [1 km] x antiP [1 km] x antiO [1 km]
  • gmimdk80gGM2RotPO [0 km] = gmimdk80g [2 km] x RotD50divGM_AR [0 km] x antiP [0 km] x antiO [0 km]