Add benchmarking framework using asv

[ianthomas23]

This PR adds a framework for benchmarking xarray-spatial and includes benchmarks for slope and viewshed for both CPU and GPU/rtxpy algorithms. It closes issue #583.

The idea is that developers will add benchmarks for other algorithms so that eventually we have full coverage.

Documentation is included in benchmarks/README.md.

Example of command-line output for the slope algorithm:

  ======= ============= =============
  --                  type           
  ------- ---------------------------
     nx       numpy          cupy    
  ======= ============= =============
    100      610±30μs    1.32±0.08ms 
    300    1.77±0.09ms   1.25±0.04ms 
    1000    14.8±0.2ms   1.52±0.02ms 
    3000     135±5ms      2.81±0.1ms 
   10000    1.28±0.03s    30.7±0.8ms 
  ======= ============= =============
  started: 2021-12-10 13:11:21, duration: 28.0s

and screenshot of the web page output:

showing that on my dev machine the cupy algorithm is ~1.5 orders of magnitude faster than the numpy one for decent problem sizes.