Improve to_string for eV and charge units

[SimonHeybrock]

• eV was formated correctly before, but meV, keV and friends were previously all formated as a 1.60217...e-XY J => format as expected.
• Ah was previously shown as 3600 C => format as Ah
• As = C, I suppose this should be kept, despite being slightly inconsistent with the above, since C is so common.
• Formatting hour as h (as in SI) instead of hr (e.g., kAh instead of kAhr) would be preferrable, but here I kept things consistent with the rest of the library. I suppose formatting options that allow configuring this are a separate discussion.

[codecov-io]

Codecov Report

Merging #112 (4d65b6b) into master (d40349c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #112   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            5         5           
  Lines         4532      4534    +2     
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+ Hits          4532      4534    +2     

[phlptp]

I think I am going to merge this. Then take a look at tweaking a few things to get rid of some of the specific strings.

I am also going to take another look at making sure the strings are SI compliant including changing the hr to h on the output.