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

pyproject.toml

@@ -15,3 +15,6 @@ ignore = [
 
 # test_smoke.py star-imports sage.all and loads a .sage file, so ruff can't see where its names come from.
 "tests/test_topdata_and_disc.py" = ["F403", "F405"]
+
+# Same star-import + load() pattern as above.
+"tests/test_period_computation.py" = ["F403", "F405"]