Rename src/histogram/accumulators.cpp to src/register_accumulators.cpp etc

[HDembinski]

It is generally a good idea to call a file after the class or function if the file only includes a single class or function. For me it would be much more logical to rename accumulators and the other implementation files to register_something so it matches the functions in these files.

[HDembinski]

It would make it easier to navigate to the right file in Atom, for example.

[henryiii]

Sure, the names of these files are not something I necessarily thought much about and should be clear/consistent.

[henryiii]

(Technically this is an "enhancement", and not a 0.50 milestone requirement, as it doesn't change the API, but it's easy and I'd like it done sooner rather than later.)