Added more tabs for conversion

Added new tabs for conversion:

1. Ounces to Grams
2. Pounds to Kilograms
3. Gallons to Liters

0.1.3

• fixed bug where inputting a decimal value will return "please input a valid number!"

0.1.2

• fixed bug where app would crash if user inputs any letter of the alphabet

0.1.1

• fixed a bug where pressing "convert" without any input would crash the app
• fixed a bug where users can input a negative amount of miles in the "miles to kilometers" tab

0.1.0

initial version, only allows Miles to Kilometers and Fahrenheit to Celsius conversion