Regtabletotext

- Introduced check if 't' column is present, otherwise use 'z' column to handle models with only one coefficient

- Replaced explicit options by flexible `**options` parameter
- Supported classes: `statsmodels.regression.linear_model.RegressionResultsWrapper` and `linearmodels.panel.result.PanelEffectsresult`
- Introduced `ALLOWED_OPTIONS` global

- First working version with statsmodels support