[ENHANCEMENT] Create class method to write samples to file

### Relation

The script, `RUN_uncor` and `RUN_terminal` are good and simple script to introduce users to the encounter model classes and how to generate model samples or tracks derived from samples. However this script nor the the `EncounterModel` and `UncorEncounterModel` classes have the functionality to write the samples or tracks to file. This functionality existed in the previous default example scripts, `RUN_1_emsample` and `RUN_2_sample2track` and functions, `em_sample` and `sample2track`.

### Description

- Add class method to `EncounterModel` to write samples to file
- Inherit (preferred) or override new `EncounterModel` method to enable `UncorEncounterModel` to write samples to file
- Inherit (preferred) or override new `EncounterModel` method to enable `CorTerminalModel` to write samples to file

Per the [convention guide](https://pages.llcad-github.llan.ll.mit.edu/airspace-encounter-models-internal/airspace-encounter-models-internal.github.io/contribute/#convention-guide), the files should be encoded in UTF-8 with linux style (`\n`) line endings

### Additional

This issue is for samples only, a issue #36 is for writing tracks to file. While there is also no `EncounterModel` superclass method for `sample`, the desired functionality for issue #35 should be easier to implement and be flexible enough to write to file based on basic model parameters.