Added physical pressure units (#5613)

* Added physical pressure units

This uses tuple pair which stores units required to be converted to respective other units as mentioned.
Available Units:- Pascal,Bar,Kilopascal,Megapascal,psi(pound per square inch),inHg(in mercury column),torr,atm

* Formatted file

File was formatted as per repo rules

* Reformatted file :)

* Added more reference

* More reference added