This function returns a set of one-dimensional intervals. The returned value
can be used in all functions that accept 'intervals' argument.
One-dimensional intervals is a data frame whose first three columns are
'chrom', 'start' and 'end'. Each row of the data frame represents a genomic
interval of the specified chromosome in the range of [start, end).
Additional columns can be presented in 1D intervals object yet these columns
must be added after the three obligatory ones.
If 'strands' argument is not 'NULL' an additional column "strand" is added
to the intervals. The possible values of a strand can be '1' (plus strand),
'-1' (minus strand) or '0' (unknown).