Rename data_index to mode_shape in Exodus reader.

Field variable mode_shape[0] represents the time step (t_index)
if HasModeShapes is not set or the ModeShape otherwise.

Change-Id: Id25aab00dfe04ae142a477650cbc3a17c9b11254
Merge topic 'exodus-15127' into master

084b0bb1 Rename data_index to mode_shape in Exodus reader.