Allow proximity search to accept lat & long at the BAO/api level & custom search form

The BAO (& by extension api) supports parameters in contact.get for proximity searching. prox_distance is a required param for this & if submitted the following parameters are used to calculate the centre point

'prox_street_address' 
'prox_city' 
'prox_postal_code' 
'prox_state_province_id'
'prox_country_id' 
'prox_state_province' ,
'prox_country' 
'prox_distance_unit' ,

I wish to make it such that those parameters are optional & not used if 'prox_geo_code_1' & 'prox_geo_code_2' are supplied. The immediate use case is to get a unit test added but I also want this to

a) improve the code in the custom proximity search so that it can use the main group filter &

b) add the UI option of specifying lat & long rather than an address where preferred (not all coordinates have an address!)

Re documentation - I guess since this is tested it is supported so I could add to the docs - at least the params I add tests for.