Label data files with extension of data format

Right now in the SystemML project, it is fairly typical for matrix data files to not have the data type extension appended to the file name. For example, running sample.dml on an input matrix will result in output files such as the following: 1, 1.mtd, 2, and 2.mtd.

As a result of this, for a user looking at the data files, a data file either (1) needs to be clicked on and opened to tell the file type, or (2) the metadata file needs to be opened to tell the data file type.

From a usability standpoint, the user should be able to tell the file type by looking at the file name. We can do this by labeling the data file with the extension of the data format.

A couple solutions to this would be:
(1) If a file extension isn't provided (in write statements), automatically label the file with the correct file extension.
(2) Rely on manual entry of the file extension. Go through the project (including tests) and make sure the generated data files include the file extensions.