Rename module files in plotting folder - #554

The plotting folder contains the most commonly used functions/modules.

Most files in that module contained a function with the same name as a file itself.
Because of the lazy-loader and how import mechanics work, we ran into the following problem:

import plopp as pp

# At this point, `pp.slicer` would point to the plotting.slicer.slicer()` function, as it should

from plopp.plotting.slicer import Slicer

# Now the name slicer has been reassigned
pp.slicer  # <-- points to the plotting.slicer module

So after importing the Slicer class, we can no longer call the pp.slicer function.

Here's what happens:

1. plopp.__init__.py uses lazy.attach to lazily expose slicer (the function) from the plotting submodule at pp.slicer.

2. When we from plopp.plotting.slicer import Slicer, Python imports the module plopp.plotting.slicer and, as part of the import machinery, sets it as an attribute on the plopp.plotting package: plopp.plotting.slicer = <module>.

3. Now when the notebook later accesses pp.slicer, the lazy loader resolves it by doing from plopp.plotting import slicer. But plopp.plotting.slicer is now the module (set in step 2), not the function. So pp.slicer becomes the module, and calling it fails with TypeError: 'module' object is not callable.

So in this PR, we rename the files in these modules to avoid the name clashes.
There are probably other files in the Plopp codebase where this also happens, but for now we only rename the most used/important modules.
We can always fix more later.

I am hoping that very little code will be broken by this change, as most code or notebooks should be importing from plopp and not from plopp.plotting.slicer.