Support SARIT, add स्कन्दपुराणम्

[epicfaace]

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• Add sarit.py to import from SARIT
• Make gretil.py more generic so its functions can be reused by sarit parsing code. In a future PR, I will move these functions to a "tei_utils.py" file.
• Add स्कन्दपुराणम् from SARIT

[akprasad]

So far so good! As part of the test plan, let's make sure the original gretil script still works.

[epicfaace]

@akprasad should be ready for review now, and GRETIL import still works.

[epicfaace]

Actually, there are two issues with skandapuranam:

1. the TEI XML file is incomplete ("The edition of the Skandapurāṇa is an ongoing project, so far the first 69 Adhyāyas have been covered. As further volumes are published this file will be extended.")
2. no xml:id attributes in the TEI XML (https://raw.githubusercontent.com/sarit/SARIT-corpus/master/skandapurana.xml):

[akprasad]

Forgive the late follow-up. At a minimum, any verse/paragraph we display should have an xml:id defined, as this is how we cross-link translations, etc.