Speed up ProcessWithContext.process_*()

[frankenjoe]

Relates to #100

Speeds up PredictWithContext.process_*() by avoiding calls to pd.concat() on a lot of small pd.Series objects. Instead, we now concatenate lists and create the final pd.Series from those.

Benchmark

import time
import audb
import audinterface


db = audb.load(
    'emodb',
    version='1.3.0',
    format='wav',
    sampling_rate=16000,
    mixdown=True,
)

def process_func(x, sr, starts, ends):
    y = []
    for start, end in zip(starts, ends):
        y.append(x[:, start:end].mean())
    return y

process = audinterface.ProcessWithContext(process_func=process_func)

t = time.time()
process.process_index(db.files)
print(time.time() - t)

• main branch: ~4.0 s
• this branch: ~0.6 s

[codecov]

Codecov Report

Merging #104 (f0a77ee) into main (7ad15b7) will not change coverage.
The diff coverage is 100.0%.

Impacted Files	Coverage Δ
audinterface/core/process_with_context.py	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

[hagenw]

As the main branch now incorporates #102 and #103 I re-did the benchmark and I get:

• main branch: ~1.9 s
• this branch: ~0.4 s