Feature to plot contour of multivariate functions

[povinsahu1909]

Fixes #128

New feature of plotting contour was added for multivariate function which was currently not supported in SympyGamma.

[povinsahu1909]

@asmeurer can you please review this pr for any improvement or the necessary changes.

[povinsahu1909]

@asmeurer Is this PR require any changes or it is ready to be merged

[zolabar]

@povinsahu1909, thank you for this contibution

@povinsahu1909, @asmeurer, @aktech, what do you think about a further extension of SymPy Gamma to multivariate functions ( at least from R^2 to R) as in the IexFinder project?

The IexFinder project is now indexed in Zenodo and can be referred to as

Zoufiné Lauer-Baré. (2021). IexFinder: Interactive Symbolic Calculation of Extreme Values (v1.0.1). Zenodo. https://doi.org/10.5281/zenodo.5707406

The IexFinder web application, powered by Heroku and Voila can be tested directly with the link

https://iexfinder.herokuapp.com/

The exFinder project is now listed in the Voila Gallery, as the fisrt SymPy project listed there

https://voila-gallery.org/

I think that SymPy Gamma has a great further potential (also compared to commercial CAS's) when going up the dimensions in calculus and in visualization!

In particular it can get ahead of Wolfram Alpha in specific cases.