LakeShore218: allow defining names for the metric

@david.leppla-weber these are the changes we discussed today - you can now add names to the metrics (and select the interval) via:

metric_names = ["TEMP_1", "TEMP_2", "TEMP_3", "TEMP_4", "TEMP_5", "TEMP_6", "TEMP_7", "TEMP_8"]
metric_interval = 5

Please let me know if this works.

TODO:

• Update README
• Do some sanity checks on metric_names and metric_interval