Add 'pview' command; fix 'check' command; added --no-missing-check;

1. ctx_pview.c: Added pview command to print entire path

2. assemble_contigs.c: added option to assemle contigs using paths. Still has bugs

3. Fixed BUG in ctx_check.c where paths were not being checked

4. graph_walker.c: added option to not use missing info check

5. path_tests.c: Added tests to check paths are added properly