fwa_locatealong returns two results when provided a measure shared by stream endpoints

[smnorris]

Function should choose just one location.
For example

# select postgisftw.fwa_locatealong(356493192, 340.150635772625);
                                  fwa_locatealong                                   
------------------------------------------------------------------------------------
 01010000E0BD0B0000F4FF7ECDEF593841BE51BB372B721F4101BC749318A07B4083500E0169427540
 01010000E0BD0B0000736891CDEF59384185EB51382B721F4101BC749318A07B4083500E0169427540
(2 rows)

# select linear_feature_id, downstream_route_measure, upstream_route_measure 
from whse_basemapping.fwa_stream_networks_sp
where blue_line_key = 356493192 and downstream_route_measure < 341
order by downstream_route_measure limit 2;

 linear_feature_id | downstream_route_measure | upstream_route_measure 
-------------------+--------------------------+------------------------
         707281883 |                        0 |     340.15127525919155
         707281606 |         340.150635772625 |      971.0342820216467
(2 rows)

[smnorris]

The results of above query relate back to #79 - the upstream measure of the downstream feature should be exactly the same as the downstream measure of the upstream feature (or at least closer than 1cm). Deriving both measures from the geom should fix this (but probably would not fix this issue).

[smnorris]

Rounding above measures to nearest cm helps but fwapg should support measures to the nearest mm - because streams really can be that short.

[smnorris]

Closing in favour of #79