Rename ModelChain test fixture `system`

[cwhanse]

This should be renamed to sapm_dc_snl_ac_sapm_temp_system follow the pattern for other fixtures.

[veronicaguo]

@cwhanse I'm happy to take a stab at this. Also, what do you think about naming it to sapm_dc_snl_ac_system? it seems to be more consistent with the other fixture names. Thanks!

[cwhanse]

Ok with me to be sapm_dc_snl_ac_system and only have the temperature model terms added when that's important for the fixture.