Add accessors to SDClass

[regenesis-art]

SDClass maintains three data members that may be of interest to callers but are not exposed. Additionally, they are declared private so could not be made available by subclassing SDClass. Present commit adds three one-line accessor functions to allow acces to these members by reference.

[PaulStoffregen]

I'm reluctant to expand the API to these (old) SdFat specific objects.

The long-term plan is to transition to the experimental optimized code. It won't implement these objects.