add 'lpg on <loc> make' and 'lpg on <loc> ensure'

[Quelklef]

Add these three commands:

• lpg on <loc> make, equivalent to lpg make <loc>

• lpg ensure <loc>, equivalent to [ -e <loc> ] || lpg make <loc>

• lpg on <loc> ensure, equivalent to lpg ensure <loc>

These are useful when using alias my-lpg="lpg on '$MY_LPG_LOC' ", so that you can just my-lpg ensure before an lpg operation to make sure it succeeds.

Maybe better is to add an -e flag to every on op, which creates the DB if it does not exist? Then you can alias my-lpg="lpg on '$MY_LPG_LOC' -e ".