Add a getAvailableCollections method in python

[jmcarcell]

BEGINRELEASENOTES

• Add a getAvailableCollections method in python that does the same thing as in C++

ENDRELEASENOTES

Currently frame.collections is the way of getting the collections. I'd prefer the C++ method and the python one to have the same name so it's easier to remember.

I'm also in favor of removing the current .collections which is probably only used in podio (in podio-dump).

[tmadlener]

No objections from my side to add this (and potentially deprecate the collections property and removing it after the next tag. Unless, you want to keep that for the "pythonic interface"?

[jmcarcell]

.collections now prints a warning and I changed it to .getAvailableCollections() where I found it. I'm fine with removing it, it's one less thing to remember or check when it's different in C++ and python.

[tmadlener]

Can we make this a warnings.warn("text", FutureWarning)? That would give a bit more context as to where the warning is coming from