New ServiceEMS for Weaver filtering of "private" processes

[fmigneault]

Context

There is a need to bloc display, access and update of "visibility": "private" processes by various routes in https://github.com/crim-ca/weaver

Previously, this was done through https://github.com/Ouranosinc/twitcher/tree/pavics-0.3.17, a Twitcher variant that defined WPS REST-API routes and Process/Job/Quote/Bill stores that aren't native to Twitcher. Now, the REST API has be completely moved to Weaver.

MagpieAdapter.processstore_factory and MagpieProcessStore provided implementations specific to EMS to do the processes filtering as defined above.
https://github.com/Ouranosinc/Magpie/blob/901a42c22578076321d8c459a3f5525fcf9848b6/magpie/adapter/magpieprocess.py

Following #182 and Weaver's creation, the process stores are deprecated.

Solution

A custom service, dynamically loaded (#149), namely ServiceEMS, would do the process filtering as required.

Dynamic ServiceEMS should be installed only when specified with pip install magpie[ems]