siground rounds to 0 when within 1e-15.

`cos(pi/2)`, for example, is not exactly 0 due to floating point
rounding error - it's 6.123233995736766e-17.

This commit changes `math.siground` to round a to 0 when `isclose(a,0)`,
which is true if it's ±1e-15.

runtime/scripts/math.js

@@ -1227,6 +1227,9 @@ var math = Numbas.math = /** @lends Numbas.math */ {
         if(a.complex) {
             return math.complex(math.siground(a.re,b),math.siground(a.im,b));
         } else {
+            if(math.isclose(a,0)) {
+                return 0;
+            }
             return parseFloat(a.toPrecision(b))
         }
     },

tests/jme-runtime.js

@@ -3448,6 +3448,9 @@ var math = Numbas.math = /** @lends Numbas.math */ {
         if(a.complex) {
             return math.complex(math.siground(a.re,b),math.siground(a.im,b));
         } else {
+            if(math.isclose(a,0)) {
+                return 0;
+            }
             return parseFloat(a.toPrecision(b))
         }
     },

tests/jme/jme-tests.mjs

@@ -1011,6 +1011,8 @@ Numbas.queueScript('jme_tests',['qunit','jme','jme-rules','jme-display','jme-cal
             assert.equal(evaluate('ceil('+expr+')').value, c, 'ceil('+expr+')');
             deepCloseEqual(assert,evaluate('fract('+expr+')'), evaluate(fr), 'fract('+expr+')');
         });
+
+        assert.equal(evaluate('siground(cos(pi/2),3)').value, 0, 'siground(cos(pi/2))');
     });
 
     QUnit.test('Currency',function(assert) {

tests/numbas-runtime.js

@@ -3449,6 +3449,9 @@ var math = Numbas.math = /** @lends Numbas.math */ {
         if(a.complex) {
             return math.complex(math.siground(a.re,b),math.siground(a.im,b));
         } else {
+            if(math.isclose(a,0)) {
+                return 0;
+            }
             return parseFloat(a.toPrecision(b))
         }
     },