Fixes #2735: Component-wise cbrt

doc/AsciiQuickReference.txt

@@ -140,6 +140,8 @@ R.array().square()             // P .^ 2
 R.array().cube()               // P .^ 3
 R.cwiseSqrt()                  // sqrt(P)
 R.array().sqrt()               // sqrt(P)
+R.cwiseCbrt()                  // cbrt(P)
+R.array().cbrt()               // cbrt(P)
 R.array().exp()                // exp(P)
 R.array().log()                // log(P)
 R.cwiseMax(P)                  // max(R, P)

doc/CoeffwiseMathFunctionsTable.dox

@@ -170,6 +170,19 @@ This also means that, unless specified, if the function \c std::foo is available
   sqrt(a[i]);</td>
   <td>SSE2, AVX (f,d)</td>
 </tr>
+<tr>
+  <td class="code">
+  \anchor cwisetable_cbrt
+  a.\link ArrayBase::cbrt cbrt\endlink(); \n
+  \link Eigen::cbrt cbrt\endlink(a);\n
+  m.\link MatrixBase::cwiseCbrt cwiseCbrt\endlink();
+  </td>
+  <td>computes cube root (\f$ \cbrt a_i \f$)</td>
+  <td class="code">
+  using <a href="http://en.cppreference.com/w/cpp/numeric/math/cbrt">std::cbrt</a>; \n
+  cbrt(a[i]);</td>
+  <td></td>
+</tr>
 <tr>
   <td class="code">
   \anchor cwisetable_rsqrt

doc/QuickReference.dox

@@ -458,6 +458,7 @@ mat1.cwiseMax(mat2)         mat1.cwiseMax(scalar)
 mat1.cwiseAbs2()
 mat1.cwiseAbs()
 mat1.cwiseSqrt()
+mat1.cwiseCbrt()
 mat1.cwiseInverse()
 mat1.cwiseProduct(mat2)
 mat1.cwiseQuotient(mat2)
@@ -470,6 +471,7 @@ mat1.array().max(mat2.array())    mat1.array().max(scalar)
 mat1.array().abs2()
 mat1.array().abs()
 mat1.array().sqrt()
+mat1.array().cbrt()
 mat1.array().inverse()
 mat1.array() * mat2.array()
 mat1.array() / mat2.array()

doc/SparseQuickReference.dox

@@ -172,6 +172,7 @@ sm2 = perm * sm1;    // Permute the columns
   sm1.cwiseMax(sm2);
   sm1.cwiseAbs();
   sm1.cwiseSqrt();
+  sm1.cwiseCbrt();
   \endcode</td>
   <td>
   sm1 and sm2 should have the same storage order

doc/snippets/Cwise_cbrt.cpp

@@ -0,0 +1,2 @@
+Array3d v(1,2,4);
+cout << v.cbrt() << endl;