Change delta_t dtype to numeric

[AdamRJensen]

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The dtype of the delta_t parameter in the solar position algorithm should be numeric as both floats, arrays, and Series can be passed.

[kandersolar]

I did not check, but I suspect we do not have any tests for Series delta_t in the SPA functions. With how fiddly numba can be, I think those tests are needed before we can confidently claim numeric.