add difference function and update unit_vector in arithmetic.hpp

[rjnrohit]

Description

1. The difference function added to arithmetic.hpp to calculate difference between two representation points(i.e r1 -r2 , where r1 ,r2 are coordinate representations).
2. Unit_vector function overloaded to calculate unit_vector directing from representations(r2) to representation1(r1)(i.e r1-r2/|r1 - r2|).
3. Tests cases have been added to verify the above implementations.

[rjnrohit]

@lpranam @sarthak2007 please review this PR and suggest necessary changes if you want any.

[sarthak2007]

Looks good to me

[lpranam]

Looks good but need to resolve the conflicts first :)

[rjnrohit]

Just give me one or two days, I've to understand the changes that have been done till now. But surely I'll be able to resolve this conflict in two days.