Added main class and unit tests

All Tests passed and basic unit.py working

>>units.runTests

test_combine (units.__UnitConverterTestCase) ... ok
test_compare (units.__UnitConverterTestCase) ... ok
test_convert (units.__UnitConverterTestCase) ... ok
test_getCompoundUnit (units.__UnitConverterTestCase) ... ok
test_getUnit (units.__UnitConverterTestCase) ... ok
test_isCompound (units.__UnitConverterTestCase) ... ok
test_setunits (units.__UnitConverterTestCase) ... ok
test_unitCompare (units.__UnitConverterTestCase) ... ok
test_unitParse (units.__UnitConverterTestCase) ... ok

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Ran 9 tests in 0.015s

OK