APIs allow repeated capabilities to access attributes and methods that don't support them

Description of issue

As of nimi-python 1.4.7, the way repeated capabilities work is thus:

1. When a _RepeatedCapabilities object, such as Session.channels, is accessed, it returns a _SessionBase object with _SessionBase.repeated_capability updated, based on what's passed to index operator [].
2. When setting an attribute or calling a method using a repeated capability, _SessionBase.repeated_capability is passed to the method or SetAttribute method.

_SessionBase is one big collection of properties and methods that support repeated capabilities, with no distinction of which repeated capabilities are supported. As an example, the nifgen API will allow a user to set an attribute that supports the data_marker repeated capability with the script_trigger repeated capability. Because we allow this, the user may be puzzled when they get back an error.

Start by reading the _RepeatedCapabilities and _SessionBase behavior described in the issue, using Session.channels and the nifgen data_marker and script_trigger examples as entry points. Determine how supported repeated capabilities are associated with attributes and methods, and consider the work done when unsupported combinations are rejected before producing a confusing API error.