Refactor BeliefsDataFrame initialization by detecting the initialization method

[Flix6x]

We currently have 5 distinct ways of initializing a BeliefsDataFrame (see below), all handled by a single __init__ function. It's not very clear which method is being supported by each code block in this function. We should detect the initialization method early on and call separate functions for each method to keep it clean. Alternatively, we could implement separate class methods for alternative constructors. This is what Geopandas (a more advanced library that subclasses pandas Series and DataFrames) does.

Initialization methods

1. from a list of timely_beliefs.TimedBelief objects
2. from a pandas.DataFrame object and a timely_beliefs.Sensor object
3. from a pandas.Series object and a timely_beliefs.Sensor object
4. from a timely_beliefs.BeliefsSeries object (undocumented)
5. from a timely_beliefs.BeliefsDataFrame object (undocumented)

And a 6th implemented as a separate function (timely_beliefs.read_csv) that uses method 2 internally.