Fix behaviour of names argument in bigWigAverageOverBed of pybigtools.

Fix behaviour of names argument in bigWigAverageOverBed of pybigtools
for `True` and `1+` as values passed to `Name::Column()` were off by one.

Before for a 4 column BED file, you would get the following error
for `True` or `4`:

    Exception: Invalid name column option. Number of columns (4) is less than the value specified (5).

pybigtools/src/lib.rs

@@ -1175,22 +1175,21 @@ fn bigWigAverageOverBed(
             Some(names) => match names.extract::<bool>(py) {
                 Ok(b) => {
                     if b {
-                        (Name::Column(4), true)
+                        (Name::Column(3), true)
                     } else {
                         (Name::None, true)
                     }
                 }
                 Err(_) => match names.extract::<isize>(py) {
                     Ok(col) => {
-                        if col < 0 {
-                            return Err(PyErr::new::<exceptions::PyException, _>(
-                                "Invalid names argument. Must be >= 0.",
-                            ));
-                        }
-                        if col == 0 {
-                            (Name::None, true)
-                        } else {
-                            (Name::Column(col as usize), true)
+                        match col {
+                            0 => (Name::None, true),
+                            1.. => (Name::Column((col - 1) as usize), true),
+                            _ => {
+                                return Err(PyErr::new::<exceptions::PyException, _>(
+                                    "Invalid names argument. Must be >= 0.",
+                                ));
+                            }
                         }
                     }
                     Err(_) => {