Update progress bar for PAR fitting (+ save loss values)

[npatki]

Description

Currently, the PAR model's verbose=True option does a good job at showing me the progress bar and the loss values. However, it is not streamlined with our ideal usage and it also isn't possible to get the loss values later.

Expected behavior

Progress bar: When verbose=True, continue to show the progress bar as usual, but with some minor modifications.

1. There is no need to show the Epoch #, as that is what the progress bar is incrementing by
2. Round the loss values to 3 decimal digits, in order to keep to consistent layout
3. Instead of showing the time per iteration/elapsed time, show the estimated time remaining instead

Save the loss values for later use: The full (unrounded) loss values should be saved for future use. They can be accessed through a parameter. Save them as a pandas.DataFrame.

>>> par.loss_values
      Epoch     Loss
0     1         -1.0191212
1     2         -1.23012421
...

Additional context

See CTGAN issue sdv-dev/SDV#298