Force pastastore name to equal pandas object name

[dbrakenhoff]

Problems arise when the name of the added pandas series (or name of the column in the case of DataFrames) does not match the name that is used in the store.

This discrepancy in names can cause weird stuff to happen when pastas StressModels are created directly from pandas objects (after being retrieved from the store). The name of the series in the model then differs from the name of the series in the pastastore. Since models and timeseries are stored separately, the name of the timeseries in the model cannot be matched to objects in the store, which causes problems.

The proposed solution is to force the name of the pandas series to match the name in the pastastore. Also removing support for multi-column DataFrames should simplify enforcing this requirement.

[dbrakenhoff]

Implemented through d0b960d

[dbrakenhoff]

Closed by #20