Add tables and functions for ground-motion field data to be used in loss-calculator

To enhance the possibilities of the loss-calculator and to make inquiries about the ground-motion values for damaged assets easier, the ground-motion field used for damage computations should be included in SpatiaLite databases that exposure-lib creates. Also, supporting functions to use these databases need to implemented:

• Create tables and indices
• Function to add ground-motion values
• Function to interpolate ground-motion values at the locations of entities/assets

\rfc @tara @laurens