Refactor 'amp' and 'hypamp' variables in Step for correct unit conversion

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (2db0f81) 70.22% compared to head (16763d1) 70.41%.

Files	Patch %	Lines
bluepyefe/recording.py	83.33%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #167 +/- ## ========================================== + Coverage 70.22% 70.41% +0.19% ========================================== Files 35 35 Lines 2223 2221 -2 ========================================== + Hits 1561 1564 +3 + Misses 662 657 -5 ```

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[ilkilic]

added the unit conversion in 'set_amplitudes_ecode'