Add Dataset close and context manager support to the Python API

[gspowley]

Add context manager support to the Python Dataset class.

import tiledbvcf

ds_uri = "..."

with tiledbvcf.Dataset(ds_uri) as ds:
    num_records = ds.count()

For code that doesn't use the context manager, a separate close method is available.

import tiledbvcf

ds_uri = "..."

ds = tiledbvcf.Dataset(ds_uri)
num_records = ds.count()
ds.close()