More context processing

[gkellogg]

• Make expandContext @type: @id and set option values to be relative to the manifest location. Fixes Base URL is not always absolute in Context Processing #369.
• Set a fallback in expand() processing if there is no documentUrl. This uses JsonLdOptions/base, which could be null, or something else appropriate for when the input or expandContext is not a URL. For Unclear how to set original base URL in Context Processing #370.
• Describe that if documentUrl is not available, values come from JsonLdOptions/base, which now defaults to null. Fixes No default value defined for base URL in Create Term Definition #375.
• Reorder clauses in 5.1.2 to make it more clear that setting base IRI and original base URL of the new active context is unconditional, unlinke previous_context.
• Create values for context document and loaded context in the case that the previous document was cached in 5.2.3 of the Create Context Algorithm. Fixes Use of possibly undefined variables in Context Processing #372.
• Describe how to handle a remote context that is not a valid IRI (not absolute) when it can't be resolved against base URL, because it is also not a valid IRI. For Possible use of non-URL values as the base URL in expand() #373.
• Update expand() to pass the original base URL from the current active context when creating a new active context using expandContext. Fixes Possible use of non-URL values as the base URL in expand() #373.
• Pass null for active property on initial call to the Expansion algorithm from expand(). Fixes Missing use of required Expansion input variable in expand() #374.

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[pchampin]

Good for me, modulo a slight change line 1256.