Move base Data types to their own files to respect plugin type convention

[sphuber]

Fixes #1098

Note that this fixes the problem for Bool, Float, Int and Str but I couldn't move the List base type as putting that in its own file list.py would cause imports to conflict with the list builtin. Not sure how to fix this and whether having its plugin type incorrect is a big problem. In general, I think the convention that node types are based on their module path is very fragile and clearly limits how we can organize our source code. We might need to find a better solution. To be discussed I would say.

The plugin_type column of the node class is used to store the class that
should be used to load the node from the database. But more importantly
this type string, which is a period separated string is used in querying
to determine class hierarchy. Nodes with similar leading parts of their
plugin_type strings fall under the same class, and when that class is
used in querying, all classes that fall below that umbrella will also be
considered.

Having the base data types Bool, Float, Int and Str not be in their own
separate file, whose module path is what determines the plugin_type, will
break this paradigm. For example, when a user queries for all nodes of
type Int, this will also return all Bool, Float and Str objects as they
all have exactly the same plugin_type string prefix. Moving these classes
back to their own file will restore this original functionality even
though it might not be the best idea to have the plugin_type string be
determined by the module path of the class itself.

[giovannipizzi]

Can we preserve in some way back-compatibility for people loading from the old place?

[sphuber]

I am not sure, because that would require importing the Int classes etc. into the aiida.orm.data.base file, but that would lead to a circular import. One way would be to move BaseType and NumericType to aiida.orm.data.__init__.py and have the Int class etc. import from there and then just use aiida.orm.data.base as a file to import the various classes. But that would still break the import for BaseType, NumericType and List.