Updates to the param classes: 
- added get_value functions to param_array and param_node
- made templated param_value::get const
- fixed is_null for param_array, _value, and _node
Added line-number search for parsing errors in param_input_json::read_file
Critical fix to mantis_server that affects absolutely nothing.
Added digitizer testing setup and new application: test_mantis_digitizer
Implemented the px1500 test (untested)