retrieve and set quantity average with cell notation

* EG: q{1} returns q(1).average and q{1} = 2 sets q(1).average
* update error messages for cos, log2 and log10
* throw error if unit indexing or assignment attempted
* if unit./quantity use left divide to send to quantity, then define left
 divide for unit using right divide
* EG: if u./q return q.\u so that it calls as_quantity, then xunit\yunit
 calls unit left divide method which is just yunit/xunit
* fix bug if units cancel out, then strjoin in combine doesn't work
 because requires string cell, so if is not string cell then return
 DIMENSIONLESS
read defaults from xml

* reads xdoc, xroot, gets units, reads attributes, creates unit, adds
 key-value pair to ureg
* use Quantities.unit.parse_name to evaluate values
* fix bugs in subsref, use only 1st s.type and s.subs, replace s(1).subs
 for aliases and break
* dimensionality of dimensionless should have been empty string!
* only validate if dimensionality is not empty
* add mtimes, mldivide, mrdivide, and power
* import constants and prefixes after base dimensions
* change power operator from ** to ^ in constants