signalSorter improvements, general git downloader, and misc updates

[bahanonu]

• signalSorter - Allow zoom and other functionality and a lock-check to prevent skipping forward to new cells based on pressing keyboard too long carrying over keypress into next source output, causing it to skip over outputs.
• signalSorter - changed how axes are called to allow more flexibility with other UI elements.
• signalSorter - Added progress bar for cell, non-cell, and unknown.
• viewAcceptedRejectedCellExtraction - additional options to improve display.
• Change additional functions that download and load external software packages to use '_external_programs' folder.
• Added a general github repo downloader, at the moment supports NoRMCorre, additional support coming.

[bahanonu]

Checked whole pipeline on Windows with MATLAB 2018b and functional (see below for ICA, CNMF, CNMF-E, and ROI analysis). Outside software now in _external_programs.

Merging.