Converts a real value from paper space units to model space units
(acet-ps-to-ms [value][viewport])
If both the value and viewport arguments are specified, the value is converted to model space units using the specified viewport. No user input is required.
If only the value argument is specified, the current viewport is assumed and no user input is required. However, if the current space is model space, there is no current viewport and the function will fail (returning nil). If paper space is the current space, the function will either prompt for a viewport if more than one viewport exists in the layout, or use the single existing viewport.
If no arguments are specified, the function prompts for a value and converts it if possible.
Arguments
A real value to be converted.
A viewport entity name (ads_name).
Return Values
The converted real value on success, nil on failure.
