[FEATURE] - Add support for multi-line terachem keywords

[ethan-r-curtis]

Some keywords in terachem are multiple lines long. For example,

run minimize 
$constraint
bond 1  2
bond  3   4
angle 1 2 3
dihedral 12 18 19 13
$end
basis 6-31g
purify no
...

This input should be parsed as 
keywords = {
    "constraints": {
        "bond": ["1 2", "3 4"],
        "angle": ["1 2 3"],
        "dihedral": ["12 18 19 13"]
    }, 
    "purify": "no"
...
}

and then passed to terachem appropriately.