ERT-769 update to use the plplot 5.10.0 library
Merge pull request #639 from iLoop2/ERT-769

ERT-769 update to use the plplot 5.10.0 library
ERT-769: Added hasWell() method
ERT-769: Added __str__() for well_segment
ERT-769: Added numSegments() method
ERT-769: Added method to get segment from index.
ERT-769: WellTImeline accepts [-1] index
ERT-769: Fixed int / offset_type bug in fortio_fseek_data()

The function fortio_fseek_data() seeks to an arbitrary location in a
file; the target offset position was stored in an integer variable. For
large files this offset wrapped around.
ERT-769: Added new test for large UNRST file.