Update CSRMatrix and push coverage to 100%, add to_uint_array util

[rly]

Motivation

With the change in hdmf-common-schema of dtypes in CSRMatrix from int to uint (hdmf-dev/hdmf-common-schema#46), the API needs to be updated in order to avoid data type conversion warnings.

Also add testing of the different error and edge cases of CSRMatrix.

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