Support IEEE time units and time precisions, #234. (#2253)

Includes `timescale, $printtimescale, $timeformat.
VL_TIME_MULTIPLIER, VL_TIME_PRECISION, VL_TIME_UNIT have been removed
and the time precision must now match the SystemC time precision.
To get closer behavior to older versions, use e.g. --timescale-override
"1ps/1ps".

src/V3Begin.cpp

@@ -135,8 +135,8 @@ private:
                     }
                     // Create CellInline for dotted var resolution
                     if (!m_ftaskp) {
-                        AstCellInline* inlinep
-                            = new AstCellInline(nodep->fileline(), m_unnamedScope, "__BEGIN__");
+                        AstCellInline* inlinep = new AstCellInline(
+                            nodep->fileline(), m_unnamedScope, "__BEGIN__", m_modp->timeunit());
                         m_modp->addInlinesp(inlinep);  // Must be parsed before any AstCells
                     }
                 }