Cache the collected gauge and clover fields when doing split grid

[hummingtree]

One could cache the collected gauge and clover fields and reuse the previously generated fields when doing split grid. The starting point should probably be

• quda/lib/interface_quda.cpp

  Line 3180 in ed21580

  quda::GaugeField *collected_gauge = nullptr;

• quda/lib/interface_quda.cpp

  Line 3202 in ed21580

  quda::CloverField *collected_clover = nullptr;

In order to do so, one needs

• A logic to determine if the previously generated field can be used.
• Then putting the cached field into a container somewhere for reusing.