Implement Data.tools.* to access methods defined by a plugin

[giovannipizzi]

Similarly to the .tools.* for Calculations (see #2322) we can implement the same concept for Data, to e.g. cover the tools that are already in aiida.tools.* (e.g. kpoint list generation, structure exporting, ...) and we could move there also other things that now live in the classes (e.g. UPF parsers etc.).

Mentioning also #2311 for the meaning of what will be in aiida.tools.

[ltalirz]

We now have an aiida.tools.data entry point

aiida-core/setup.json

Lines 211 to 214 in 9ff07c1

	"aiida.tools.data.orbitals": [
	"orbital = aiida.tools.data.orbital.orbital:Orbital",
	"realhydrogen = aiida.tools.data.orbital.realhydrogen:RealhydrogenOrbital"
	],

@giovannipizzi If that is not what you had in mind (it's not entirely clear to me from the description), feel free to reopen.