fix: prevent integer overflows with numpy 2.0

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 14.28571% with 12 lines in your changes missing coverage. Please review.

Project coverage is 69.34%. Comparing base (7431e63) to head (2ca0be7). Report is 1 commits behind head on main.

Files	Patch %	Lines
pyTMD/io/OTIS.py	16.66%	10 Missing :warning:
pyTMD/spatial.py	0.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #304 +/- ## ========================================== + Coverage 69.03% 69.34% +0.31% ========================================== Files 38 39 +1 Lines 8657 8844 +187 Branches 1171 1195 +24 ========================================== + Hits 5976 6133 +157 - Misses 2308 2325 +17 - Partials 373 386 +13 ``` | [Flag](https://app.codecov.io/gh/tsutterley/pyTMD/pull/304/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tyler+Sutterley) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/tsutterley/pyTMD/pull/304/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tyler+Sutterley) | `69.34% <14.28%> (+0.31%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tyler+Sutterley#carryforward-flags-in-the-pull-request-comment) to find out more.

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