Implement remaining methods for in-memory add_group_number

[GregoryTravis]

..Equal_Sum Field:Column NumberOfGroups:Integer=5 OrderBy:...

..Equal_StDev Field:Column <Step=1> OrderBy:...

Find the mean, find the standard deviation.

 Number of groups must be an odd number.
 Start from -Step/2 to +Step/2 (group starting group)
     Spread outwards with negative group number

 Option to add a set of names (standardised): Average, Above/Below Average, High/Low, Very High/Very Low, Extremely High/Extremely ...

..Custom
field:Column
<upper_bound:Vector x>
<add_group_names:Boolean=FALSE>

Document if is inclusive or exclusive bound (lower out, upper in)

Equal_Sum support the order_by.
Standard_Deviation and Custom_Ranges will error if order_by given as will be ignored.

Equal_Sum find the target sum for each group and then scan record in until each set exceed target. Last group will be undersized. It can result in less target groups if actually achieving sum is impossible without basically one group.