Units

This MR introduces Units into miniflask.
Those are custom numbers that share multiple representations.

TODO:

• check syntax for better simplicity. it seems to me that more complicated units can be defined very good currently, but easier ones (like time, with a fixed set of conversions) are cumbersome to define. Can we do better?
• add unit tests
• add examples to documentation

Example

First, we define time to be a unit with four different representations.:

def convert_time_unit(src_value, src_unit, dest_unit):
    factors = {
        "second": 1,  # base unit
        "minute": 60,
        "hour": 60 * 60,
        "day": 60 * 60 * 24,
    }

    # first convert to base, then to dest unit
    return src_value * factors[src_unit] / factors[dest_unit]

time = mf.register_unit("time", convert_time_unit, [
    ["second", "s"],
    ["minute", "m"],
    ["hour", "h"],
    ["day", "d"]
])

In CLI, we can overwrite this unit using:

• --time 1.5d (1.5 days)
• --time 24h (24 hour)
• --time 500m (500 minutes)

Using u in v Notation, we can specify the base unit to convert all calculations to.:

• --time 12h in d (0.5 days)
• --time 1.5d in h (18 hours)

To register a unit, pass it to register_defaults as follows:
Note, that the base unit is always the one that is passed last to the variable.
In the example below, time uses hours internally, time2 and time3 use days internally.

mf.register_defaults({
    "time": time(6.0, "h"),
    "time2": time(6.0, "h in d"), # in notation works here as well
    "time3": time(6.0, "d")
})

Querying other representations can be done easily, once defined:

state["time"].unit # the base unit name
state["time"].second
state["time"].minute
state["time"].hour
state["time"].day

Calculations works similarly:

state["time"].second += 10
state["time"].minute += 10
state["time"].hour += 10
state["time"].day += 10

Things done in this MR

• added internal classes for units (Unit, UnitValue, UnitValueArgparse)
• added Units to Argparse / CLI-Parsing
• added mf.register_unit funciton

Check all before creating this PR:

• Documentation adapted
• unit tests adapted / created