Fixed issue with Linear Units and added FromURN

[WillGunther]

Description

• I was having some some segfaults while testing my WMTS stuff and calling LinearUnits. Based on the docs I think that the pointer we are giving the function is being overwritten by a pointer to something in C land, and then we are trying to free it. This shouldn't be a memory leak as long as the SR is destroyed.
• Added a from URN convenience function. I spent some time looking for it because it is listed in gdal docs, but it turns out there is no binding for it because SetFromUserInput does the same thing

Testing

unit testing

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 5.77%. Comparing base (9019316) to head (3d18c09).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #32 +/- ## ======================================== + Coverage 5.57% 5.77% +0.20% ======================================== Files 7 12 +5 Lines 3158 3324 +166 ======================================== + Hits 176 192 +16 - Misses 2975 3125 +150 Partials 7 7 ```

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