UNST-9277: support mulitple ext filenames mdu

[thomas-pijls]

What was done

We want to support that extForceFileNew contains multiple *.ext filenames in a space-separated list.

• modified init_new subroutine to process multiple *.ext files at once
• modified findexternalboundarypoints subroutine to handle multiple *.ext files
• small cleanup & reformatting
• created new testcase to test multiple *.ext file input (e02_f005_c160_multiple_extfiles)
• documentation updated in User Manual and D-Hydro file specifications

TODO

• Purple, FEWS, and OpenDA teams notified

Evidence of the work done

• Implemented code
• Purple, FEWS, and OpenDA teams notified

Tests

• Test added

Documentation

• User Manual appendix A and C6.2 updated
• D-Hydro file specifications updated

Issue link