Could we make expression parsing more explicit on UCUM codes vs aliases

[stevegeek]

See discussion/issues:

• Unitwise::Unit.new("atm") fails #57
• Area are with deci prefix fails #26
• Miles per hour #70 - adding mph would conflict with milli phot
• Inconsistent symbol for compound unit with bit, bit_s (and pH/[pH]) #81 (possibly)

PRs:

• update: prioritise atomic units to prefixed units #56
  -Prioritise atomic units to prefixed units  #82

Note that UCUM has a grammar definision, that seems inconsistent?

https://ucum.org/ucum

<simple-unit> | ::= | <ATOM-SYMBOL>\| <PREFIX-SYMBOL><ATOM-SYMBOL[metric]>

while https://ucum.org/docs/formal-grammar

simpleUnit :  prefix metricAtom // prefix is token from lexer
           |  metricAtom  
           |  nonMetricAtom // token from lexer
           |  DIGITS    // allow zero?

[stevegeek]

The change from prefix symbol taking priority over non prefixed was here? ucum-org/ucum@041d9bb

Was:

<tr><td><sym name="prefix"/></td><td>::=</td>
    <td><sym name="PREFIX-SYMBOL"/></td></tr>
  <tr><td><sym name="proper-atomic-unit"/></td><td>::=</td>
    <td>  <sym name="ATOM-SYMBOL"/><br/>
        | <sym name="ATOM-SYMBOL"/> <sym name="annotation"/>
	</td></tr>
  <tr><td><sym name="atomic-unit"/></td><td>::=</td>
    <td>  <sym name="proper-atomic-unit"/><br/>
        | <sym name="annotation"/></td></tr>
  <tr><td><sym name="annotation"/></td><td>::=</td>
    <td><lit value="{"/> <sym name="ANNOTATION-STRING"/> 
          <lit value="}"/></td></tr>
  <tr><td><sym name="simple-unit"/></td><td>::=</td>
    <td><sym name="prefix"/><sym name="proper-atomic-unit"/><br/>
        | <sym name="atomic-unit"/><br/>
	| <lit value="("/> <sym name="term"/> <lit value=")"/><br/>
        | <sym name="factor"/></td></tr>
  <tr><td><sym name="power"/></td><td>::=</td>