Fixes some issues in a few functions (one fix per commit), namely:

• Use proper type assertion in ttext_start_value and floatset_value_n
• Remove random return in floatset_start_value
• Remove casts from Datum to to proper return values in nad functions (nad_tnumber_number returns double directly)
• Allow the conversion of TPoints in tgeometry-tgeography conversion functions
• Allow the use of any temporal value as a base temporal framework for t[geo|point]_from_base