Add startup variable selection with -var option

[cmorency1]

This PR adds a -var <variable_name> command-line option so NcVis can automatically select and load a requested variable at startup.

What changed:

• Added parsing for -var in command-line options
• Stored the requested startup variable in wxNcVisFrame
• Auto-selected the requested variable after the variable selectors are populated
• Triggered the normal variable selection flow so startup loading uses existing logic
• Updated usage text to document the new option

Behavior:

• If the requested variable exists, it is automatically loaded on startup
• If it is not found, NcVis shows a warning and falls back to normal behavior