Speedup import time with lazy imports

[mataton]

Addresses #984

The average import time (python -X importtime -c 'import biom') over 7 runs for the current master branch of BIOM is 0.44s. Moving the imports of the required dependencies into their respective functions brings the average import time down to 0.19s, a 2.3x speedup.

Performance of individual functions is only impacted on the first call. For example, the first call to Table.to_dataframe() is 247 microseconds, while all subsequent calls are 180 microseconds. This is due to the module being cached in sys.modules after the first call. The execution time of to_dataframe() without the import statement inside the function is 180 microseconds.

[wasade]

Thanks @mataton! Would you be able to add a brief note to the change log? I'll merge after

[mataton]

@wasade Thank you for the reminder!