ENH: reduce memory usage when applying geo operations on large input files

[theroggy]

Evade having out-of-memories, regardless of the size of the input files, eg. by:

• limit batch size for geopandas based operations
• give the user an option to specify batch size explicitly for cases where the heuristics don't work out
• avoid using group_by clauses in queries, as this seems to use a lot of memory
  • erase/union/identity
  • export_by_location
  • clip
• have more intelligent determination of the size of batches in general
• check memory usage not only when starting an operation, but also eg. before the start of the processing of a new batch.
• the sqlite cache size is now quite large, test if it can be reduced without impacting throughput (too much)

[theroggy]

Memory usage for geopandas based operations was reduced here: 0ede76f