create class for storing/writing metropolis draws

In the while loop of matlab/posterior_sampler_core.m we have an example of how draws are stored and written when a certain number of draws have been stored in memory.

Need to create a Matlab class that stores:

• a vector
• a matrix
• a structure (dr)

And that writes itself to disk when a certain number of vector/matrix/structures have been written and clears itself so that more draws can be stored.

This class can then be used to standardize the various ways we do this throughout the Matlab codebase.