gain_groups: Add zero-value gain groups

Add convenience factory for making a gain group that has a single
zero-valued element. This factory requires a name, which should
probably be ALL_GAINS, or something similar (these constants are
device-specific).

Using this new make_zero factory in the X300 radio control when we
don't find any gain elements so that our gain groups aren't empty. This
simplifies our later setters/getters because we know that we'll always
have _something_ cached.
Note that we only register this zero value gain group for TX, as our
ADC is registered as a gain element, so our RX gain groups are never
empty.

host/include/uhd/utils/gain_group.hpp

@@ -91,6 +91,13 @@ public:
      * \return a gain group object.
      */
     static sptr make(void);
+
+    /*!
+     * Make a new gain group with all zero values.
+     * \param name the name of the (only and zero-valued) gain element
+     * \return a gain group object populated with zeroes
+     */
+    static sptr make_zero();
 };
 
 } // namespace uhd

host/lib/utils/gain_group.cpp

@@ -182,3 +182,14 @@ private:
 gain_group::sptr gain_group::make(void){
     return sptr(new gain_group_impl());
 }
+
+gain_group::sptr gain_group::make_zero()
+{
+    gain_fcns_t gain_fcns;
+    gain_fcns.get_range = []() { return meta_range_t(0.0, 0.0); };
+    gain_fcns.get_value = []() { return 0.0; };
+    gain_fcns.set_value = [](const double) {};
+    auto gg             = make();
+    gg->register_fcns("null", gain_fcns);
+    return gg;
+}