MEX Drivers and Tests (#297)

* First round of changes for mex test.

* Updatd mex test and refactored image reading code.

* Merged testing and refactored code.

* First round of changes based off of comments.

* Updated tests to use load function.

* Updated method documentation for reading the image data.

* Fixed typo and removed unused imports.

* Driver for MEX and Tests for MEX Driver

* Address review comments, part one - testing push to branch

* Update testing re: comments and hopefully fix failing tests

* Added documentation to new properties and removed isis_bytes property as it was unused.

* Add parent instrument_id check