Echoregions

What's Changed
* Using Regionmask to mask bottom by ctuguinay in https://github.com/OSOceanAcoustics/echoregions/pull/198
* This treats the bottom masking as more of a 'region mask' than that of an 'above/below line mask'
* Add functionality for saving to `.evl` and `.evr` files by ctuguinay in https://github.com/OSOceanAcoustics/echoregions/pull/199
* Fix left edge behavior for `.bottom_mask` by ctuguinay in https://github.com/OSOceanAcoustics/echoregions/pull/201

**Full Changelog**: https://github.com/OSOceanAcoustics/echoregions/compare/v0.2.0...v0.2.1

General Code Changes:

- Added Feature to read from and write to `Regions2D` and `Lines` DataFrames 148
- Return bottom and region points when masking
- Rework `select_sonar` to make use of `ds_Sv` calibrated Sv Echopype data products instead of `.raw` filenames 169
- Reworked transect checking component of `transect_mask` to be faster and less strict in its transect checking
- Allows transect mask to be created without start and end transect values

To be included in Later Releases:

- Add functionality in `Lines.mask` for masking using `regionmask`
- `Regions2D` DataFrame -> `evr` functionality
- Re-implement Combine `Regions2D` Object

This is the initial release of Echoregions: A lightweight package designed for processing region and bottom line labels from Echoview, and creating masks on [echopype](https://echopype.readthedocs.io/en/stable/)-generated backscattering data.

Initial Features

- Processing Echoview Line (EVL) and Echoview Region (EVR) Files
- EVL files generally contain both human-annotated and Echoview-annotated depth data
- EVR files generally contain region annotations for different marine organisms and survey logs
- Creating 2D and 3D region masks for echopype-generated Sv datasets based on depth and time values in the EVL and EVR files
- Subsetting, plotting, and saving processed EVL and EVR data

To be included in later releases

- Merging of EVR file data
- Reading of EVL and EVR files from an S3 Bucket