Some proposed name changes

[raylutz]

There are a few somewhat ambiguous names that have been used by other dataframe packages. These, we can either adopt their confusing names for consistency, or adopt better names. These are under discussion.

old name	proposed new name	rationale
cols	colnames	this is a list of names of the columns, not the entire column
.columns()	.colnames()	same rationale
keyfield	rowkeycol	explains that this is for row selection
.keys()	.rowkeys()	keys() normally selects dict items, and would be comparable to colnames()
.hd	.cd	column dictionary vs. header dictionary.
.kd	.rowkd	row key dictionary rowkd vs. key dictionary.

[raylutz]

krows and kcols is now being used in select_kcols and select_krows to refer to names used to select rows and columns by name. irows, icols used for referring to indexes.

[raylutz]

Put aside for now.