Xer-reader

Added `to_excel` function. Export the tables in the .xer file to individual spreadsheets in a .xlsx file.

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* Added docstrings for attributes.
* Fixed typo in README.

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Access to `Table` class.

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General code clean up.

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Added

* Added `currency` attribute pulled from ERMHDR values.
* Added `get_table_names` method.
* Added `has_table` method.
* Added `parse_tables` method. Replaces the `tables` attribute.

Removed

* Removed `tables` attribute. The tables are now returned in the `parse_tables` method.

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Fixed typo in README example.

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