Allow loading and verification of multiple variables within the same execution

This is also a precursor to computing multivariable indicators. This development depends on the new as.s2dv_cube() function (See #48 (closed))

TODOs:

• Modify Loading module Start() call to load multiple variables.
• Adapt Loading module to change prlr units in multivar case
• Modify data_summary() to loop over each variable and display the correct information
• Check Calibration module
• Check Skill module
• Check Anomalies module
• Adapt Saving module to loop over each variable
• Adapt Visualization module to loop over each variable
• Testing