Initial setup for period computation (#13)

Introduces:
 - class for Picard--Fuchs operators and their ideals
 - class for periods (their complex linear combinations in the Frobenius bases)

Implements:
 - method to obtain operator from period
 - method to get power series solution (at given indicials) to PF ideal

---------

Co-authored-by: Julian Piribauer <julian.piribauer@gmail.com>
Reviewed-on: #13

sage/util.py

@@ -0,0 +1,15 @@
+"""
+Utility functions for the Calabi-Yau period geometry project.
+"""
+
+def _multi_indices(no_variables, total_degree: int) -> list:
+    # All no_variables-tuples of non-negative integers whose entries sum to at most total_degree.
+    def helper(remaining_vars, remaining_degree):
+        if remaining_vars == 0:
+            yield ()
+            return
+        for first in range(remaining_degree + 1):
+            for rest in helper(remaining_vars - 1, remaining_degree - first):
+                yield (first,) + rest
+
+    return list(helper(no_variables, total_degree))