Script Generator: Import an excel sheet

As a SANS scientist, I would like to be able to write out actions in an excel spreadsheet and load them into the script generator using that spreadsheets file.

## Acceptance criteria:

- [ ] We have defined a file structure of the excel sheet
- [ ] Users can import an excel spreadsheet to fill out the script generator
- [ ] Users can specify the script definition to use in an importable excel spreadsheet
- [ ] If the given script definition cannot be found, the user is given the option to select one and the script generator attempts to load the actions table and global parameters
- [ ] Users can specify values for global parameters in an importable excel spreadsheet
- [ ] Global parameters are identified by their label in an importable excel spreadsheet
- [ ] Users can specify values for the actions table in an importable excel spreadsheet
- [ ] Columns are identified by the column name in an importable excel spreadsheet
- [ ] Any empty cells in the columns of an importable excel spreadsheet are left blank in the script generator

## Notes:

- This is a requirement from the SANS scientists to use the script generator
- Following this SANS require https://github.com/ISISComputingGroup/IBEX/issues/6750