This posts contains summary of two weeks work.

I was working on introducing transformation equations in `CoordSysCartesian`

.
We decide, up to now, to connect every other curvilinear coordinate system to
Cartesian, so equations maps the rectangular one to the desired one. We allow
user to set new coordinate system in three possible way:

So we can set coordinate system by `str`

from pre-defined `dict`

. The second way
is to do it by equations, where variables are base scalars. The last possibility
is to create tuple which contains two tuples. In first, there should be variables
which we want to use to define transformation equations, and in the second one
transformation equation itself.

During this time I’ve created several PR, which corresponds directly to the
transformation of coordinate system #12803, #128822, #12872, but I also updated documentation #12871 and refactor some code. So now we have `CoordSys3D`

instead of
`CoordSysCartesian`

#12829. Updated documentation went into 1.1 release of SymPy.
It was necessary to avoid mismatch between the code which was previously updated
or deprecated and documentation