Addition of Laplacian operation

[mohit3011]

Fixes #12381

[jksuom]

This appears related to #12261.

[mohit3011]

@jksuom , then would you consider merging the changes made by me?

[asmeurer]

I agree this looks a lot like the other PR. I would likely prefer the other one as it is older, but also I prefer the changes slightly (it calls the function laplacian instead of laplace, also "Laplacian" is a different concept from "Laplace transform").

[mohit3011]

@jksuom , @asmeurer , thanks for reviewing it. I think I will close this PR.

[mohit3011]

I just wanted to know, why there is an error coming in doc tests. Also, I ran the code quality tests on my terminal then they were running fine but here in Travis test, these are giving error.
Please explain. Thanks.

[jksuom]

bin/doctest was failing because laplace was not imported in its doctstring examples (https://travis-ci.org/sympy/sympy/jobs/213513081).

[mohit3011]

So , does that mean that every time I include some examples (like here , I included Examples of Laplace in vector/functions.py) I have to include the function name of functions in docstrings too ?

[jksuom]

When doctest is running the examples in docstrings, such as this one, for example,

    >>> from sympy.vector import CoordSysCartesian, divergence
    >>> R = CoordSysCartesian('R')
    >>> v1 = R.x**2*R.y**2*R.z**2
    >>> laplace(v1, R)
    2*R.x**2*R.y**2 + 2*R.x**2*R.z**2 + 2*R.y**2*R.z**2
    >>> v2 = 2*R.y**2*R.z
    >>> laplace(v2, R)

it simply copies each >>> line verbatim and feeds it to the Python interpreter. It does not automatically import the function. Therefore everything has to be given in the same way as when running the examples manually.

[mohit3011]

Ok , Thanks @jksuom