Update the MAX7219 to test for bad data input.

[mholtrop]

Some routines, originally intended for internal use only but not underscored, do not check if the input is within acceptable limits. This should be tested so that programming errors are caught.

Specifically: WriteLocChar, WriteRaw, WriteInt need guards.

Also, WriteData's argument "data" is confusingly close to self.DATA. Change name.