Statprocon

- Add helper method `is_lnpl_above_floor()` to check if the Lower Natural Process Limit has any value that is above the specified floor argument. This can be used to check if LNPL should be used or not
- Add optional parameter `lower_natural_process_limit_floor` to `x_to_dict()` method. If the LNPL is not above this value, it will not be included in the result
- Remove optional `floor` parameter from `lower_natural_process_limit()`. The helper method can be used to check if this condition occurs

- Rename `moving_average()` to `x_moving_average()`

- Add optional `moving_average_points` parameter to `to_dict()` and `x_to_dict()` to return moving average values for the number of points

- Add optional `floor` parameter to `lower_natural_process_limit()` to allow specifying a value that LNPL cannot be more negative than

- Add optional `include_halfway_lines` parameter to `to_dict()` and `x_to_dict()` to return halfway lines that should capture 85% of X values in predictable processes

- Add `XmRTrending()` class to compute trending X central line and natural process limits