Add some documentation

documentation/numeric_types.md

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+# Mystery numbers
+
+Sis data uses some numeric classifiers that are not self-explanatory and (most) previously needed to be deciphered from existing code.
+Some mystery numbers are gathered here for convenience. Feel free to add anything that could be useful. Remember to use the spelling used in code to enable project wide search.
+
+## extentcode
+
+(from **studyright**)
+
+These are actually defined in db table studyright_extents. Tells studyright level or type, eg. bachelor's degree or non-degree studies.
+
+## prioritycode
+
+(from **studyright**)
+
+1: Primary
+
+2: Secondary
+
+5: Rescinded (Seemingly not in use anymore)
+
+6: Option
+
+30: Graduated
+
+## type
+
+(from **element_details**)
+
+20: A study programme
+
+30: Module/Studytrack/kokonaisuus? Eg. Neurology
+
+## enrollmenttype
+
+(from **semester_enrollments**)
+
+1: Present
+
+2: Absent
+
+3: Inactive (neither 1 or 2 has been declared)
+
+## credittypecode
+
+(from **credit**)
+
+Defined in credit_types db table. Completed/Improved/Transferred/Failed