server: reduce csv value from fixture to value
Server: compute route accepts namespace

Substitute symbols in expression from namespace provided by client
server: add new dataset to server tests
server: tests renamed df -> accounts
server: test that from_tree is robust to unnecessary namespace
server: non-dataset-specific server
server: tree namespace uses symbols, not dshapes
server: Client uses new compute.json route
doc: update server docs to use compute.json
Merge pull request #770 from mrocklin/server-non-dataset-specific

Server compute route is not specific to one dataset