Group index step validators

[dbochkov-flexcompute]

Current group index validator doesn't error on ModeSpec(group_index_step=1) https://github.com/flexcompute/tidy3d/blob/develop/tidy3d/components/mode.py#L133-L140. From what I understand, this happens because 1 is automatically converted into True given the field definition group_index_step: Union[bool, pd.PositiveFloat]. I tried changing it to group_index_step: Union[pd.PositiveFloat, bool] but in this case True gets converted into 1. Not sure what would be the best way to resolve this issue. I guess one option is to just split this field into two :

• group_index_step: pd.PositiveFloat = pd.Field(GROUP_INDEX_STEP, ...)
• compute_group_index: bool = pd.Field(False, ...)

What do you think, @lucas-flexcompute?

Also, root validator https://github.com/flexcompute/tidy3d/blob/develop/tidy3d/components/mode.py#L142 probably needs skip_on_failure=True. Otherwise, even if previous validators fail, it still gets executed and fails itself when trying to get values["group_index_step"]. So, instead of desired

           ERROR: Parameter 'group_index_step' must be less than 1.  mode.py:140

what gets displayed is:

>       if values["group_index_step"] > 0:
E       KeyError: 'group_index_step'

tidy3d/components/mode.py:146: KeyError

[lucas-flexcompute]

Fixed in #1152