`from_ms()` can't deal with negative growth rates in exponential form

[grahamgower]

$ python -c "import demes; demes.from_ms('2 1 -t 1.0 -G -1e-5', N0=1)"
Traceback (most recent call last):
  File "/usr/lib/python3.9/argparse.py", line 1851, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib/python3.9/argparse.py", line 2060, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/usr/lib/python3.9/argparse.py", line 1990, in consume_optional
    arg_count = match_argument(action, selected_patterns)
  File "/usr/lib/python3.9/argparse.py", line 2155, in _match_argument
    raise ArgumentError(action, msg)
argparse.ArgumentError: argument --growth-rate/-G: expected 1 argument

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/grg/.local/lib/python3.9/site-packages/demes/ms.py", line 828, in from_ms
    args, unknown = parse_ms_args(command)
  File "/home/grg/.local/lib/python3.9/site-packages/demes/ms.py", line 430, in parse_ms_args
    args, unknown = parser.parse_known_args(command.split())
  File "/usr/lib/python3.9/argparse.py", line 1854, in parse_known_args
    self.error(str(err))
  File "/home/grg/.local/lib/python3.9/site-packages/demes/ms.py", line 28, in error
    raise ValueError(str(exc))
ValueError: argument --growth-rate/-G: expected 1 argument

See fix for analogous problem in msprime's mspms: tskit-dev/msprime#1722