Fix link types

[schlegelp]

I just tried loading connectors from the VFB CATMAID:

>>> import pymaid
>>> rm = pymaid.CatmaidInstance(server="https://fafb.catmaid.virtualflybrain.org/", api_token=None)
>>> cn = pymaid.get_connectors(1875104)
>>> cn.type.value_counts()
type
unknown     856
Synaptic    118

Turns out that the link ID -> type map hard-coded in pymaid.config is different for that CATMAID instance.

This PR changes three things to fix the issue:

1. New function, pymaid.config.get_link_types(remote_instance), that fetches the map and caches it for subsequent use.
2. Adds a custom __hash__ method for CatmaidInstance that generates the hash based on server url, token, project ID and http user/password. This is so that changing any of the relevant properties will also change the hash and lead to re-caching of the link map (and possibly other stuff in the future).
3. The link map contains for each connector type a name and type (plus a bunch of other variables). Previously we used the type to populate the corresponding column in the dataframe returned by get_connectors. However, on the VFB CATMAID instance we have:

   [
   {'relation_id': 14, 'name': 'Presynaptic', 'type': 'Synaptic', ...},
   {'relation_id': 17, 'name': 'Postsynaptic', 'type': 'Synaptic', ...}
   ]

   I.e. the type is the same for both pre- and postsynapses. With this PR we're using the name field instead.

@clbarnes I know you're out of the game technically but so am I when it comes to CATMAID. I also don't have access to your internal CATMAID instances, so can't test there. Do you see any reason why the changes suggested there might cause problems?

[clbarnes]

I think you're right to do this as technically it can differ between instances and hardcoding database IDs is always a risky business; this should be correct. Note that for most endpoints, the returned linked type is not the raw database ID, but a canonicalised version which puts the synaptic relations in 0, 1; gap junction in 2 etc.. I never remember which way round they are, here it is in the source https://github.com/catmaid/CATMAID/blob/2964e04e6e9772aff5d305e72c1b878030fe0e25/django/applications/catmaid/control/skeletonexport.py#L247

Good idea on the hashing for the cache!