Import

* Refactored `Import` in a way that makes developing import converters easier (i.e. written in the Mathematica language)

* Implemented `Text` and `CSV` formats. Still working on `JSON` format.

* Implemented some miscellaneous helper functions: `Partition`, `StringSplit`, `StringReplace`, `ToExpression`. 

* Refactored `FileFormat` to use `mimetypes` from the standard library. 

Outstanding Issues :
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* Context of ```RegisterImport``` should be ```ImportExport` ``` so that it is accessed using ```ImportExport`RegisterImport```