Changed Float duration to be configureable for hrs+days

[menningd]

In many mobile application is is desirable to keep the BMS in float mode for longer periods of time. This allows charge sources to supply loads without cycling the battery. Eventually it must still end so that the charge cycle can restart and allow cells to rebalance. See further discussion in issue #310 . The format of the timer has been changed from minutes to days+hrs. It allows 15minute increments with a max total time of 28 days, 23.75hrs.