More work on test_variable.py
Completed Variable test coverage
Completed file separator
Early work on main read() function
Implemented extract() function
Initial work on Model class and parse.read()
New test program for Model class
Added main test
Reworked Model test and rearranged some read code
Partial implementation of read function
Added skeleton for filter functions
Corrected class methods and test case
Added more tests
Completed parse code
Removed old test file and copied in SIM model
Started work on code generator
Implemented test for (as-yet-unwritten) code generator
Early work on code generator
Tidied some code and reworked code handling from input file
Implemented lag() function