Added support for variable exposure rate and HRSC (#25)

* Added HRSC ISD

* Changed integration time in line scan state data.

* Added support for changed line scan state in line scan plugin

* Finished implemented HRSC support

* Fixed ctx state test file

* Fixed a bug where ls state data would have an extra int time entry.

* Added state file for HRSC test

* Added simple HRSC initialization test

* fixed tab spacing in ctx test state

* Fixed half line timing issue with CTX line scan model

* Fixed HRSC test ISD

* Removed image flip from line scan time calculations

* Fixed timing in line scan model

* Fixed timing issue with constant scan rate cameras

* Turned on HRSC test

* Updated HRSC state JSON file

* Added HRSC plugin testing

* Fixed typo in pixel to camera x, y calculation.

* Added HRSC ground to image tests

* Added new HRSC test data

* Added better HRSC tests. Now does forward, aft, and nadir.

* Moved HRSC test fixtures into genericls test file.