feat(vtk): improve vtk pathline export

[wpbonelli]

• support PRT pathline format
• support dataframe or recarray
• support merged or separate paths
• attach layer array to exported points
• motivated by https://github.com/MODFLOW-USGS/modflow6-examples/pull/161

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 26.66667% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 73.0%. Comparing base (18014af) to head (935d963).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #2132 +/- ## ========================================= - Coverage 73.1% 73.0% -0.1% ========================================= Files 260 260 Lines 59439 59466 +27 ========================================= + Hits 43454 43461 +7 - Misses 15985 16005 +20 ``` | [Files](https://app.codecov.io/gh/modflowpy/flopy/pull/2132?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy) | Coverage Δ | | |---|---|---| | [flopy/export/vtk.py](https://app.codecov.io/gh/modflowpy/flopy/pull/2132?src=pr&el=tree&filepath=flopy%2Fexport%2Fvtk.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy#diff-ZmxvcHkvZXhwb3J0L3Z0ay5weQ==) | `77.9% <26.6%> (-2.0%)` | :arrow_down: | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/modflowpy/flopy/pull/2132/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modflowpy)