New functions and improvements

[rolfsimoes]

introduces new functions:
- sits_labels() # returns a simple vector
- sits_labels() <- labels # labels names vector -- redefines the labels names
- sits_bands() <- bands # bands names vector -- redefines the bands names
- sits_foreach() # iterates over a group of data
- sits_summary() # equivalent to the older sits_labels() function
- sits_samples() # resamples sits data

modified functions:
- sits_envelope() # change parameter: adds operations, removes window.size
- sits_cluster_cleaner() # change parameter: adds min_lab_perc

removed functions:
- sits_labels_list()
- sits_labels_)