feat(legacy/io): add Abaqus keyword registration globals

Restore mutable keyword registry used by the `.inp` parser inside Legacy.

- Implement `register_abaqus_keyword` / `is_abaqus_keyword_registered`.

src/legacy/io/keyword_register.jl

@@ -0,0 +1,24 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/AbaqusReader.jl/blob/master/LICENSE
+
+global __register__ = Set{String}()
+
+"""
+    register_abaqus_keyword(keyword::String)
+
+Add ABAQUS keyword `s` to register. That is, after registration every time
+keyword show up in `.inp` file a new section is started
+"""
+function register_abaqus_keyword(keyword::String)
+    push!(__register__, keyword)
+    return Type{Val{Symbol(keyword)}}
+end
+
+"""
+    is_abaqus_keyword_registered(keyword::String)
+
+Return true/false is ABAQUS keyword registered.
+"""
+function is_abaqus_keyword_registered(keyword::String)
+    return keyword in __register__
+end