Docstring for VariantData needs writing

[hyanwong]

At the moment if you do help(tsinfer.VariantData), you get the correct Initi signature described, but the rest of the docstring is wrong (is uses the one for SampleData):

Init signature:
tsinfer.VariantData(
    path,
    ancestral_allele,
    *,
    sample_mask=None,
    site_mask=None,
    sites_time=None,
)
Docstring:     
SampleData(sequence_length=0, *, path=None, num_flush_threads=0,     compressor=DEFAULT_COMPRESSOR, chunk_size=1024, max_file_size=None)

Class representing input sample data used for inference.
See sample data file format :ref:`specifications <sec_file_formats_samples>`
for details on the structure of this file.

[hyanwong]

Fixed in #963