feat: adding mddl_xx series of macros (and tests). Closes #166

ddl/mddl_dc_maxkeytable.sas

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+/**
+  @file
+  @brief Maxkeytable DDL
+  @details For storing the maximum retained key information.  Used
+      by mp_retainedkey.sas
+
+**/
+
+
+%macro mddl_dc_maxkeytable(libds=WORK.MAXKEYTABLE);
+
+  proc sql;
+  create table &libds(
+      keytable varchar(41) label='Base table in libref.dataset format',
+      keycolumn char(32) format=$32.
+        label='The Retained key field containing the key values.',
+      max_key num label=
+        'Integer representing current max RK or SK value in the KEYTABLE',
+      processed_dttm num format=E8601DT26.6
+        label='Datetime this value was last updated',
+    constraint pk_mpe_maxkeyvalues
+        primary key(keytable));
+
+%mend mddl_dc_maxkeytable;