fixes for special horizontal TDEs halfspace

[hvasbath]

• properly migrate "is_halfspace" flag to setup_tde function for HS disp and strain
• added tests for horizontal TDE

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.26 :tada:

Comparison is base (bf7aa3e) 71.16% compared to head (c970d99) 71.42%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #27 +/- ## ========================================== + Coverage 71.16% 71.42% +0.26% ========================================== Files 19 19 Lines 1200 1211 +11 ========================================== + Hits 854 865 +11 Misses 346 346 ``` | [Impacted Files](https://app.codecov.io/gh/tbenthompson/cutde/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ben+Thompson) | Coverage Δ | | |---|---|---| | [tests/test\_tde.py](https://app.codecov.io/gh/tbenthompson/cutde/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ben+Thompson#diff-dGVzdHMvdGVzdF90ZGUucHk=) | `99.09% <100.00%> (+0.04%)` | :arrow_up: |

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[tbenthompson]

Nice! Thanks!