Refactor and document operators and periods in sage module (#21)

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Co-authored-by: Julian Piribauer <julian.piribauer@gmail.com>
Reviewed-on: #21

README.md

@@ -84,11 +84,11 @@ Note that for Calabi–Yau dimensions larger than four, the additional
 
 ## Operators and periods
 
-`operators.sage` and `period.sage` provide classes for working with Picard–Fuchs operators and their
+`operators.sage` and `periods.sage` provide classes for working with Picard–Fuchs operators and their
 period solutions: `PFOperator`, `PFIdeal` and `Period`, together with Ansatz variants of the first and
 last (`PFOperatorAnsatz`, `PeriodAnsatz`) used to search for unknown operators or periods of a given
-z- and theta-degree. The operator classes live in `operators.sage`, the period classes in `period.sage`;
-since the two depend on each other, load `period.sage`, which also loads `operators.sage`.
+z- and theta-degree. The operator classes live in `operators.sage`, the period classes in `periods.sage`;
+since the two depend on each other, load `periods.sage`, which also loads `operators.sage`.
 
 A `PFOperator` is parsed from a string in the variables `z0, ..., z<n-1>` and `theta0, ..., theta<n-1>`,
 the logarithmic derivatives theta_i = z_i d/dz_i. For example, the quintic's Picard&ndash;Fuchs operator: