Enable Creating Multiple TimeSeries

[Alrobbertz]

For instruments that collect multiple sensor measurements, at different time cadences, they require multiple time axes to be present in the same HermesData object and CDF file. This is regulated by each of the different measurement's having unique DEPEND_0 metadata attributes that reference their respective time axes with the appropriate cadence.

To achieve this we need to make updates to the HermesData container to allow for multiple time axes with different cadences, or time deltas. Currently the HermesData class only supports on "Epoch" variable encapsulated in the timeseries attribute. This needs to be extended, or another data structure added so that multiple "Epoch" variables can be added to the same data container and written to the same CDF.

[Alrobbertz]

@ehsteve How would you feel if we had something like this:

class HermesData:

  def __init__(timeseries, ..., meta):
    if isinstance(timeseries, TimeSeries):
      self._timeseries = {"Epoch": timeseries.copy()}
    elif isinstance(timeseries, dict):
      self._timeseries = deepcopy(timeseries)
      
  @property
  def timeseries(self):
    if len(self._timeseries.keys()) == 1: # Only one Epoch
      return self.timeseries["Epoch"]
    else:
      return self._timeseries

This keeps the existing API functionality in-tact, but allows us to group all of the TimeSeries structures together for easier handling and IO.

[Alrobbertz]

I started a draft PR #114