Integrates Timeslices, change time-series to PU

[ktehranchi]

Changes and Fixes:

• Integrated timeslcies with new timeslice and timeslice_value fields from plexosdb query and associated parsing logic
• Renames variables to be more intuitive. Now variables and fields for max_active_power/active_power/rating match the Sienna definitions of these terms instead of plexos definition.
• Changes export of max_active_power to be per-unit
• Fixes read-order of date_from / date_to filtering which caused all retiring units to previously not be included in system
• Refactors self._get_value()
• Fix improve: Print Missing fields when component skipped #25
• Fix Bug: Exporter Components with ',' comma in name #31

[pesap]

Why is this rating?

[ktehranchi]

I changed this to rating since active_power in Sienna only refers to the initial set point of active power (MW). Although rating in Sienna is (MVA), I still think rating makes more sense than active power since we're assuming these plexos models are not running models with reactive power.

I considered using max_active_power, but I reserved that for the plexos Rating field since that is typically used to specify time-varying max_active_power limits in combination with the Rating Factor field.

[pesap]

LGTM!