Added physical pressure units

[SabariGanesh-K]

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

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
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[SabariGanesh-K]

I did 2 conversion algorithm for the past 1 week . First one has been merged and this is the second one.
I am opening separate PR for this as it was mentioned in the rule to open separate PR for different algorithms.
@cclauss The tests are passing:) If you have any suggestions/Changes you may convey , else you may merge it !!

[cclauss]

Is there are a separate source of conversions on the internet that can confirm our results?

[SabariGanesh-K]

I have a website , but that's not wikipedia. Shall I add it ? https://msestudent.com/what-are-the-units-of-pressure/
This is the website. @cclauss

[SabariGanesh-K]

I have added reference website which has relevant unitary data .

[SabariGanesh-K]

Alternatively you can google corresponding unitary value (like 1Pa to atm ) to evaluate multiplier needed

[cclauss]

I was not referring to our unitary values. I was referring to going to a website that does conversions and then putting in our doctest values to see if their conversions match ours.

[SabariGanesh-K]

I guess to do that you may visit https://www.unitconverters.net/pressure-converter.html

[SabariGanesh-K]

@cclauss I have added website in reference which helps in converting between pressure units directly like how you asked.

[cclauss]

doctest results match well with unitconverters.net