add voltsperbit and channel ids to ecube metadata

[leoscholl]

[michaelli66]

also add an option to load voltage by multiplying out the voltageperbits

[leoscholl]

also note that currently all the channels will be loaded from all the headstages. this may not be the desired behavior in the future when we're running experiments with multiple headstages or multiple experiments simultaneously. need to also rethink the way load_ecube_metadata() and load_ecube_data() work so that they include information about which headstage to load

[leoscholl]

would be useful to add saving the xml data including voltsperbit to aopy.utils.save_test_signal_ecube()

[leoscholl]

this has turned into multiple issues. i will make some new ones. to close this issue, please focus on writing a function to load XML data (there is some in tests/data/2021-04-07_BMI3D_te1315 and tests/data/short headstage test)