Update the load_data method of the database readers

[wenh06]

An optional argument return_fs is added to the method load_data, by setting which to be True, the returned valued of load_data has signature

Tuple[np.ndarray, Real]

and the elements of the tuple are

data : numpy.ndarray
    The ECG data loaded from the record,
    with given `units` and `data_format`.
data_fs : numbers.Real, optional
    Sampling frequency of the output signal.

The default value for return_fs is False, so that the default behaviour of load_data is unchanged.

[codecov]

Codecov Report

Merging #11 (9207284) into master (8a5c9bf) will increase coverage by 0.03%.
The diff coverage is 98.11%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
+ Coverage   92.97%   93.01%   +0.03%     
==========================================
  Files         136      136              
  Lines       18115    18155      +40     
==========================================
+ Hits        16843    16887      +44     
+ Misses       1272     1268       -4     

Files Changed	Coverage Δ
torch_ecg/databases/base.py	89.54% <90.00%> (-0.06%)	⬇️
torch_ecg/databases/cpsc_databases/cpsc2018.py	96.29% <100.00%> (+0.06%)	⬆️
torch_ecg/databases/cpsc_databases/cpsc2019.py	96.85% <100.00%> (+0.06%)	⬆️
torch_ecg/databases/cpsc_databases/cpsc2020.py	98.05% <100.00%> (+0.03%)	⬆️
torch_ecg/databases/nsrr_databases/shhs.py	92.51% <100.00%> (+0.02%)	⬆️
torch_ecg/databases/other_databases/cachet_cadb.py	89.32% <100.00%> (+0.15%)	⬆️
torch_ecg/databases/other_databases/sph.py	88.12% <100.00%> (+0.10%)	⬆️
...rch_ecg/databases/physionet_databases/apnea_ecg.py	94.89% <100.00%> (ø)
...orch_ecg/databases/physionet_databases/cinc2018.py	93.33% <100.00%> (+0.22%)	⬆️
...orch_ecg/databases/physionet_databases/cinc2020.py	92.83% <100.00%> (+0.04%)	⬆️
... and 2 more

... and 1 file with indirect coverage changes