Add ability to set module-wide defaults for silencing, backend aggregation algorithm

[ks905383]

Main changes

• Implements a way to set global defaults, updates tests, docs to match

Now, xa.set_defaults() can be used, either on its own or in a with block, to set silent=True/False or impl='for_loop'/'dot_product'.

Now truly fixes #55

Minor changes

• .gitignore now ignores temporary files created during export tests