Add utils module and functions

Related to #194 (closed)

b_asic/utils.py

+"""B-ASIC Utils."""
+
+from typing import List, Sequence
+
+from b_asic.types import Num
+
+
+def interleave(*args) -> List[Num]:
+    """
+    Interleave a number of arrays.
+
+    For the input ``interleave([1, 2], [3, 4])``, return ``[1, 2, 3, 4]``.
+
+    Parameters
+    ----------
+    *args : a number of arrays
+        Arrays to interleave. Must be of the same length.
+
+    Returns
+    -------
+
+
+    """
+    return [val for tup in zip(*args) for val in tup]
+
+
+def downsample(a: Sequence[Num], factor: int, phase: int = 0) -> List[Num]:
+    """
+    Downsample a sequence with an integer factor.
+
+    Keeps every *factor* value, starting with *phase*.
+
+    Parameters
+    ----------
+    a : array
+        The array to downsample.
+    factor : int
+        The factor to downsample with.
+    phase : int, default: 0
+        The phase of the downsampling.
+
+    Returns
+    -------
+
+
+    """
+    return a[phase::factor]