Read files from FTP server

[schlegelp]

With this PR we can load neurons straight from FTP servers. For example:

# URL for supplemental data from Gowens et al. (2020)
url = "ftp://download.brainlib.org:8811/biccn/zeng/pseq/morph/200526"
# Load the first 20 SWC files matching the `_raw.swc` pattern
nl = navis.read_swc(url + "*_raw.swc", limit=20)

TODOs:

• update I/O tutorials
• update changelog