replace time with obsmjd in obstable calculation to avoid error about…

[mi-dai]

time column was assigned in ZTFObsTable (when converting obsdate to obsmjd, which will cause error when using a new colmap that include time. Changed to obsmjd.

[github-actions]

Before [2e47852]	After [d0b7cbd]	Ratio	Benchmark (Parameter)
23.8±3ms	19.0±0.6ms	~0.80	benchmarks.TimeSuite.time_chained_evaluate_sed
539±8μs	542±9μs	1.01	benchmarks.TimeSuite.time_apply_passbands
7.54±0.04ms	7.58±0.07ms	1.01	benchmarks.TimeSuite.time_apply_white_noise
19.3±0.4ms	19.5±0.2ms	1.01	benchmarks.TimeSuite.time_make_and_evaluate_static_sed
11.0±0.08ms	11.1±0.08ms	1.01	benchmarks.TimeSuite.time_make_evaluate_constant_sed_model
47.8±0.6μs	48.2±0.5μs	1.01	benchmarks.TimeSuite.time_make_new_salt3_model
33.7±0.3ms	33.7±0.2ms	1.00	benchmarks.TimeSuite.time_additive_multi_model_source
119±4μs	119±8μs	1.00	benchmarks.TimeSuite.time_evaluate_simple_linear_wavelength_model
89.1±1μs	89.5±1μs	1.00	benchmarks.TimeSuite.time_make_and_evaluate_step_model
30.3±0.5μs	30.4±0.3μs	1.00	benchmarks.TimeSuite.time_make_simple_linear_wavelength_model

Click here to view all benchmarks.

[jeremykubica]

This is going to give an error during the tests because there is already a mapping in _default_colnames doing this mapping.

What is the underlying problem? Are you trying to read a ZTF table that has a time column (with values you don't want)?

[mi-dai]

I'm trying to map time to another column (a slightly different ZTFObsTable with different columns). The old code calculates times before doing the mapping and errors when doing the mapping.