Improve transmission efficiency input data

# :rocket:

- [ ] Is your feature request essential for your project?

### Describe the workflow you want to enable
Currently, each transmission line has a pre-defined efficiency value, based on its voltage [see here](https://github.com/Breakthrough-Energy/SwitchWrapper/blob/db7a36a99a03fb99f4da197c1afa0c367cc035ac/switchwrapper/const.py#L90). I wish these values were dependent on line length as well.

### Describe your proposed implementation
Within the `switchwrapper.grid_to_switch` module, the `build_aclines` and `build_dclines` functions should be modified to look at the length of the line, and use this information with per-mile loss information stored in `switchwrapper.const` to calculate line-specific loss values.

### Describe alternatives you've considered, if relevant
Alternatively, we could prompt the user for these values.