add MapData class for creating input data file

So far is simple replacement for 'make_file' function, but will be used
to write data in chunks (for large data sets).
remove make_file function
put dataset names in global variables
change convert_data to read and writes data in chunks

Read/write data detector-by-detector instead of all at once. Necessary for
large data sets.
use argparse for option parsing in convert_data
add verbose option to convert_data

prints status messages, useful for very large datasets
Update README.md

Update description of convert_data