Since the FLAC format has a hard limit of eight channels, we need to
use multiple signal files if a record contains more than eight
signals.

Commit d1d26ba was meant to do this automatically (when
Record.set_default or wfdb.wrsamp is used to generate the signal file
names), but this was never tested.

Handle more than 8 compressed signals in wrsamp

We should be able to read and write data with more than 8 channels
to formats 508, 516, and 524. This test verifies that the digital signal
generated when writing to format 516 is identical to the original signal
loaded in from a sample record in format 16.

Add a test to check if wrsamp can write more than 8 channels to format 516

The alias 'np.int' has been removed from recent versions of numpy, and
there is no particular reason to want to use it here (as an array data
type, 'int' refers to a particular platform-dependent type, which is
not at all the same thing as an ordinary Python integer.)

Use int64 here for inter-platform consistency and to keep it simple.

Use int64 instead of int for ann2rr

Bump version to 4.1.2. Add release notes to docs.