Assemblers overhaul

[cmhamel]

Make local element scratch array type optional

• Static arrays
• Regular arrays
• CPU multi-threaded cache
• GPU cache - this will be big

Different cache types

• CPU - serial
• CPU - multi-threaded
• GPU

Remove storage from structs

• Create method that creates scratch entries for residual and stiffness
• Need to break storage out of sparse matrix assembler #213