Regtabletotext

- Added note that table contains t-statistics
- `prettify_result()` now passes lists to `prettify_results()`

- Replaced reporting of F-statistic nan with "F-statistic not available"

- Increased default `max_width` from 64 to 80

- Added support for list of `linearmodels.panel.result.PanelEffectsresult` results

- Added support for 'arch.univariate.base.ARCHModelResult'
- Simplified and extended `calculate_residuals_statistics()`
- Added covariance type and fixed effects table to `linearmodels.panel.result.PanelEffectsresult` print result
- Moved default values to function definitions
- Updated required install to pandas only
- Added support for `arch.univariate.base.ARCHModelResult`

- Moved type checks to functions and globals
- Removed unnecessary whitespace from model formula
- Introduced `max_width` with default to 64 characters