make stdev optional input, change variance to parameter input

* variance overrides stdev if both provided, no warning
* fix propagated variance for division and subtraction terms added since
 (-1)^2 = 1, duh!
* in to_base() check if already in base units, then return input as output
fix addition/subtraction quantity operations

* units don't add silly! convert to bases, then add values, use common
 base unit
* assert power exponent must be dimensionless
* assert sine/cosine must be angles
* assert log must be dimensionless
* validate unit inputs
* comment out subsref and subsasgn
* refactor operations
* try using gt/lt operators
fix bugs, any(), log, infinite loops