n_beats not needed in carat.features.feature_map()

[mrocamora]

There is no need to provide the number of beats per bar as an input parameter of the function carat.features.feature_map(). This can be easily computed inside the function from the relation between the number of beats and the number of downbeats provided, as:

n_beats = int(round(beats.size/downbeats.size))

This is a simple fix. However, it changes the function signature and impacts into several parts of the code, in particular in some examples.