Nastran long support

[Spectre5]

I've added a new test with a valid Nastran grid in long format that currently fails. The issue is that chunks2[1:3]] in the code below will only be length 1 since the second line in the test grid card is not long enough (not more than 16 characters long). This leads leads to ValueError: not enough values to unpack (expected 2, got 1).

            chunks2 = _chunk_line(next_line)
            next_line = f.readline()
            points.append(
                [
                    _nastran_string_to_float(i + j)
                    for i, j in [chunks[5:7], chunks[7:9], chunks2[1:3]]
                ]
            )

The solution I've added here is to simply use 72 for the line length instead of len(line), which will result in the fixed format line "chunks" always having 9 fields. It fills the empty ones with empty strings, ''.

[nschloe]

Very nice, thank you!