config refactor (#1854)

* split out config

* add tests for base and app config, refactor client config out of app config

* refactor default config retrieval

* create config test class and helper functions

* move default_config into server to fix import issue

server/test/fixtures/server_config_outline.py

@@ -0,0 +1,62 @@
+f"""server:
+  app:
+    verbose: {verbose}
+    debug: {debug}
+    host: {host}
+    port: {port}
+    open_browser: {open_browser}
+    force_https: {force_https}
+    flask_secret_key: {flask_secret_key}
+    generate_cache_control_headers: {generate_cache_control_headers}
+    server_timing_headers: {server_timing_headers}
+    csp_directives: {csp_directives}
+    api_base_url: {api_base_url}
+    web_base_url: {web_base_url}
+  authentication:
+    type: {auth_type}
+    params_oauth:
+      oauth_api_base_url: {oauth_api_base_url}
+      client_id: {client_id}
+      client_secret: {client_secret}
+      jwt_decode_options: {jwt_decode_options}
+      session_cookie:  {session_cookie}
+      cookie:  {cookie}
+
+  multi_dataset:
+    dataroot: {dataroot}
+    index: {index}
+    allowed_matrix_types: {allowed_matrix_types}
+    matrix_cache:
+      max_datasets: {max_cached_datasets}
+      timelimit_s: {timelimit_s}
+
+  single_dataset:
+    datapath: {dataset_datapath}
+    obs_names: {obs_names}
+    var_names: {var_names}
+    about: {about}
+    title: {title}
+
+  diffexp:
+    alg_cxg: # number of threads to use is computed from: min(max_workers, cpu_multipler * cpu_count)
+      max_workers: {diffexp_max_workers}
+      cpu_multiplier: {cpu_multiplier}
+      target_workunit: {target_workunit}  # The target number of matrix elements that are evaluated in one thread.
+
+  data_locator:
+    s3:
+      region_name: {data_locater_region_name}
+
+  adaptor:
+    cxg_adaptor:
+      tiledb_ctx:
+        sm.tile_cache_size:  {cxg_tile_cache_size}
+        sm.num_reader_threads:  {cxg_num_reader_threads}
+
+    anndata_adaptor:
+      backed: {anndata_backed}
+
+  limits:
+    column_request_max: {column_request_max}
+    diffexp_cellcount_max: {diffexp_cellcount_max}
+"""