RomNumStr can be a single string or multiple strings, either upper or lower case, joined by mathematical operators. 
RealNum can be any real number. 

Real numbers cannot be combined with Roman numeral strings. 

Return value: 
This function returns a string of Roman numerals if given a real number, or a real number if given a strings of Roman numerals. 

Examples: 
1.  RomNum("xxvi"+"L") 
     76 

2. RomNum(1999) 
     "MCMXCIX" 

3. RomNum(log("M")) 
    3