Clarifies LabJack differential analog input API via struct.
Linux LabJackScaffold uses bit-shift & bitwise-or for bit packing.
Linux LabJackScaffold can now do long settling or quick sampling for U3.
See the U3 User's Guide to explain long settling and quick sampling.
Refactors Linux LabJackScaffold.
Splits helper functions into files for checksums and type converters.
Adds a constants header file to hold the maximum buffer size.
Puts all the Linux LabJack helper functions into the LabJack namespace.
Refactors the low-level reading and checksumming into a function.
Bugfixes for LabJack setup.
Only cache output indices on successful pull, otherwise the cached
indices are all -1.
Only update if output indices have been cached or there is no
outputProducer.  Otherwise the update will try to use the indeterminate
initial values in the cache.
Device asserts on initialization if an OutputProducer is necessary but
missing.
LabJackTypeConverters cpplint fixes (const and whitespace).