Gromacs parser cannot handle incomplete lines with -

[xiki-tempula]

For an incomplete line like

1 -100 -10 -

The last - will be giving problems as it cannot be cast as a float during
https://github.com/xiki-tempula/alchemlyb/blob/093ad87eca736f724c9a4b4d6843043ea9861f69/src/alchemlyb/parsing/gmx.py#L308

where dtype=np.float64 would enforce it to be a float and - will give

File "/Users/XXX/opt/miniconda3/envs/mda_dev/lib/python3.7/site-packages/pandas/io/parsers.py", line 610, in read_csv
    return _read(filepath_or_buffer, kwds)
  File "/Users/XXX/opt/miniconda3/envs/mda_dev/lib/python3.7/site-packages/pandas/io/parsers.py", line 468, in _read
    return parser.read(nrows)
  File "/Users/XXX/opt/miniconda3/envs/mda_dev/lib/python3.7/site-packages/pandas/io/parsers.py", line 1057, in read
    index, columns, col_dict = self._engine.read(nrows)
  File "/Users/XXX/opt/miniconda3/envs/mda_dev/lib/python3.7/site-packages/pandas/io/parsers.py", line 2061, in read
    data = self._reader.read(nrows)
  File "pandas/_libs/parsers.pyx", line 756, in pandas._libs.parsers.TextReader.read
  File "pandas/_libs/parsers.pyx", line 771, in pandas._libs.parsers.TextReader._read_low_memory
  File "pandas/_libs/parsers.pyx", line 850, in pandas._libs.parsers.TextReader._read_rows
  File "pandas/_libs/parsers.pyx", line 982, in pandas._libs.parsers.TextReader._convert_column_data
  File "pandas/_libs/parsers.pyx", line 1056, in pandas._libs.parsers.TextReader._convert_tokens
ValueError: could not convert string to float: '-'