bugfix in `tests/dem_error` and `ifgram_inversion` while handling array/scalar types (#1455)

+ `tests/dem_error`: use `np.allclose` instead of `math.isclose` to handle array types

+ `ifgram_inversion.estimate_timeseries()`: Fix ValueError where inv_quality[idx] assignment fails when processing single pixels. The issue manifests in environments with stricter NumPy type checking, e.g.,
Python 3.12.12 and NumPy 2.4.1 during Circle CI testing.