Resolve "Add resampled responses as functions of the TPSs"

Closes #16 (closed)

• Use integers for SC and LINK/MOSA indices
• Add argument t to plot methods
• Rename "tt" to "ltt" for light travel times (for consistency)
• Require link array for GWResponse.compute_gw_response()
• Allow for multi-dimensional t vectors in GWResponse.compute_gw_response()
• Add argument timeframe to GWResponse.write()
• Write TPS responses (if necessary) in GWResponse.write()