Shares, a pound amount and true/false variables declare the wrong unit #2362

A variable's unit is served in the model metadata and decides how the app formats a value (/1 as a percentage, currency-GBP in pounds). In policyengine-core it also sets the default quantity_type: a float with unit = "/1" is a stock and any other float is a flow, and a yearly flow read for a month is divided by 12.

These variables declare a unit that does not match what they return:

• shareholding and consumption_shareholding return a household's share of a national total but declare unit = GBP.
• free_tv_licence_value returns pounds (the licence fee times the discount) but declares unit = "/1".
• Seven true/false variables declare a unit: sdlt_liable, lbtt_liable, benunit_is_rent_liable, is_pension_credit_eligible, is_guarantee_credit_eligible and is_savings_credit_eligible (GBP), and car_is_ved_exempt (/1).

Found by the independent specification of the tax-head partition written for #2235 (its defect 8). #2147 also corrects the free_tv_licence_value unit, for #2148.