Explicit/Implicit TrainingData read access to data

[tylertownsend]

In implicit and explicit training data, the parameters of x and y (or dx_dt for implicit) are passed to the constructor to create a new instance. However, the constructor verifies that the data is shaped correctly. Because the member variables are exposed for both read and write access, this reshaping can be skipped and the data structure can be broken. Add read only properties to the values.

[gbomarito]

finally got around to implementing this, thanks