Phoebe

* fixes version of flask-socketio dependency to remain compatible with desktop client
* ensures path and query string are cast to string

* all fitting ld_coeffs. Each coefficient is referenced by index and can be fit or have distributions attached independently. See [tutorial](http://phoebe-project.org/docs/latest/tutorials/fitting_ld_coeffs) for more details.
* also fixes support for [custom constraints](http://phoebe-project.org/docs/latest/tutorials/constraints_custom) which can be used to link ld_coeffs between datasets of the same passband, for example.

* do not require passing solution to adopt_solution (when adopting distributions) if only one solution exists
* fix distribution_overwrite_all not defined error

* excludes datasets not supported in fitting (mesh, orb, lp, etc) from forward-model within inverse solvers.
* run_checks_solver now checks for nans in dataset arrays.

* fixes bug where passing overwrite to add_compute or add_solver raised an error if run_compute/run_solver already created a model/solution tagged with that same label.

* allows disabling multiprocessing (or lowering the number of available processors). Multiprocessing is used by default when not within MPI and when calling `run_compute` with `sample_from` or `run_solver` with solvers that support parallelization. Some installations of multiprocessing on Mac may cause issues, in which case you can now for PHOEBE to run in serial mode.
* this introduces new `phoebe.multiprocessing_off()`, `phoebe.multiprocessing_on()`, `phoebe.multiprocessing_get_nprocs()`, and `phoebe.multiprocessing_set_nprocs(n)` functions, but the default behavior remains unchanged.