Adding inverse trigonometry functions

Aim: To add inverse trigonometry functions namely asin, acos & atan **Work Done:**

• Added buttons for all 3 in function pad itself as there was enough room to accommodate all 3.
• Added functionality for all 3 functions.
• Tested the result and it works without errors.
  **Issues to be resolved:**
• The functions return the value in radians but I haven't changed them as degree/radian functionality could be implemented in kalk later on. This would make any hard-coding in the present situation unnecessary.