Use a map for register definitions

[tvannoy]

Currently, register definition code looks like

mp.register(2).name       = 'rate';
mp.register(2).value      = 1;          
mp.register(2).min        = 0.1;          
mp.register(2).max        = 5;          
mp.register(2).default    = 50;          

which is fine. However, referencing these register settings in the simulink model might be easier if we use a map instead:

mp.register('rate').value = 1;
mp.register('rate').min = 0.1;

In conjunction with #4, this would make using the register data types in the model easier, especially when manually setting output data types of arithmetic blocks.