handling level=0 in waverec/wavedec functions

[grlee77]

I found an inconsistency in all wavedec functions relative to their waverec counterparts. If the signal is too small for the chosen wavelet, level will be set to 0 in wavedec and a length 1 coeffs_list containing only the original data will be returned. If this is then passed in to waverec, an error will occur, because it only allows coefficient lists of length >= 2.

There are two potential solutions here:
1.) raise an error in _check_level if level = 0.
2.) modify waverecn to allow length 1 coefficient arrays and just return coeffs[0] (which is the original data) if the coeffs input was length 1.

I would lean toward solution 2, possibly adding a warning in wavedecn and waverecn if level=0.

[grlee77]

closing this as I was mistaken and the waverecn transform` does already work as expected for a zero-level transform.

This is tested indirectly in test_multilevel.py where some tests involve large wavelets such as coif17 that exceed the test signal size resulting in a zero-level transform.