_storage_scheme -> _default_storage_scheme

[rijobro]

fixes https://github.com/SyneRBI/SIRF/issues/715.

@evgueni-ovtchinnikov how about this. Change _storage_scheme to _default_storage_scheme. If using the static method, return the default. However, if the object exists, return the corresponding type.

I've added the test, which fails with the old code, but passes with these changes:

    AcquisitionData.set_storage_scheme("memory")
    ad = AcquisitionData(raw_data_file)
    AcquisitionData.set_storage_scheme("file")
    test.check_if_equal("memory", ad.get_storage_scheme())