fits: lazily create ColFitsStarTable per-column Inputs

In ColFitsStarTable, create the per-column input objects only
when they are required, rather than pre-emptively for all columns,
in the RowSequence and RowAccess implementations.
Although this introduces an extra null test into each cell read
operation, this does not seem to impact the performance,
and it means that columns never needed are never (e.g.) mapped.

fits/src/main/uk/ac/starlink/fits/ColFitsStarTable.java

@@ -764,15 +764,20 @@ private class ColFitsRowSequence implements RowSequence {
             seqColReaders_ = new ColumnReader[ ncol_ ];
             cursors_ = new long[ ncol_ ];
             for ( int icol = 0; icol < ncol_; icol++ ) {
-                final BasicInput input =
-                    inputFacts_[ icol ].createInput( true );
+                final InputFactory inputFact = inputFacts_[ icol ];
                 seqColReaders_[ icol ] =
                         new ColumnReader( valReaders_[ icol ] ) {
-                    protected BasicInput getInput() {
-                        return input;
+                    BasicInput input_;
+                    protected BasicInput getInput() throws IOException {
+                        if ( input_ == null ) {
+                            input_ = inputFact.createInput( true );
+                        }
+                        return input_;
                     }
                     public void close() throws IOException {
-                        input.close();
+                        if ( input_ != null ) {
+                            input_.close();
+                        }
                     }
                 };
                 cursors_[ icol ] = -1;
@@ -827,14 +832,19 @@ private class ColFitsRowAccess implements RowAccess {
         ColFitsRowAccess() throws IOException {
             colReaders_ = new ColumnReader[ ncol_ ];
             for ( int icol = 0; icol < ncol_; icol++ ) {
-                final BasicInput input =
-                    inputFacts_[ icol ].createInput( false );
+                final InputFactory inFact = inputFacts_[ icol ];
                 colReaders_[ icol ] = new ColumnReader( valReaders_[ icol ] ) {
-                    protected BasicInput getInput() {
-                        return input;
+                    BasicInput input_;
+                    protected BasicInput getInput() throws IOException {
+                        if ( input_ == null ) {
+                            input_ = inFact.createInput( false );
+                        }
+                        return input_;
                     }
                     public void close() throws IOException {
-                        input.close();
+                        if ( input_ != null ) {
+                            input_.close();
+                        }
                     }
                 };
             }
@@ -938,7 +948,7 @@ private static abstract class ColumnReader implements Closeable {
          *
          * @return  basic input
          */
-        protected abstract BasicInput getInput();
+        protected abstract BasicInput getInput() throws IOException;
 
         /**
          * Positions ready to read the value for a given row.