Allow frequency strings for more functions?

[jonmd]

Allow users to use strings rather than Frequency references in many cases, such as when converting a period-based series to a DataFrame:

# Earlier:
from energyquantified.time import Frequency
df = periodseries.to_dataframe(frequency=Frequency.PT1H)

# Allow this also:
df = periodseries.to_dataframe(frequency="PT1H")