Xer-reader

Corrections to README.

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* Reference [Issue 2](https://github.com/jjCode01/xer-reader/issues/2)
* The `to_dict` method no longer throws an error if an unreckognized table is found; it now just ignores the table.
* Added `FINTMPL` to table_data.
* Updated `to_csv` method:
* Can now pass a list of table names to export to CSV. For example pass in agrument `table_names=["TASK", "PROJWBS"]` to only export the task and wbs tables to CSV.
* Can now change the delimeter. For example, pass in argument `delimeter=","` to use a coma rather than the default tab.

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Clean up code.

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Patched potential errors when transformming from a string to a float. Some languages use a comma rather than a period in floating point numbers. [Issue 1](https://github.com/jjCode01/xer-reader/issues/1)

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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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