-Rewrote the function get_unit_size() to deal with "Z64" and "Z128"

-Now written in C. Added /gstlal-calibration/tests/lal_logicalundersample_test.py for testing. Also small change to lal_constantupsample.

- Included lal_constantupsample_test.py for testing, updated test_common.py in gstlal-calibration to match what is in gstlal.

- no longer required

this makes the paths to header files in #include directives the same for
code both in and out of tree.  this helps make it possible to subclass
gstlal elements in other packages since the element header files can now be
used out-of-tree, and it makes it eaiser to move code from other packages
into gstlal core (e.g. the framecpp plugin)

- remove unused src_size argument from undersample() worker functions
- add assert to check for buffer overruns in undersample() worker functions

- remove unused *remainder argument from undersample() worker functions
- re-arrange arithmetic in transform_size() method to prevent compiler-optimization-induced integer arithmetic bugs