check if tieseries is regularly sampled

Just a tiny feature add:

• functions, that depend on whether a timeseries is harmonized or not, evaluate the index.freqstr property. Turns out, this property is not always reliable (didnt investigate into the details - but encountered examples that break). As a fix, i added a checking function to the lib.tools library and evaluated the result of this function instead of the index.freqstr property.

• while implementing the above mentioned func, i stumbled over a tiny bug in rolling.roll, that i did fix. (variable wasnt instanciated when certain if-clause wasnt satisfied).

please approve