Resolve inconsistency between `SSPData._fields` and `DEFAULT_SSP_KEYS`

[aphearin]

In #116 we temporarily set DEFAULT_SSP_KEYS to exclude emission lines, which helped resolved failing tests in diffsky. But the next release of DSPS should probably include emission lines by default, and so this should be changed back once we restore consistency to the load_ssp_templates wrapper function in diffsky.