check_functions: add options, refactor

The parsing of gsw_check_functions.m has been broken out into a function.
The script now takes options:
```
usage: check_functions.py [-h] [--path MFILEDIR] [-v] [--find FIND]

Run checks from gsw_check_functions.m

optional arguments:
  -h, --help       show this help message and exit
  --path MFILEDIR  location of gsw_check_functions.m
  -v, --verbose    print output mismatch arrays
  --find FIND      run functions with this substring
```