bitwidth limited to 31bits

[vanh souvanlasy]

it appears that libecbufr also has a 31 bits limit for anything other than strings type (CCITT IA5).
Tests done with descriptor like 24013 where bitwidth=32 shows values are not encoded or decoded properly
when values reached the limit of 31 bits.

[vanh souvanlasy (vanh-souvanlasy)]

tests on descriptor 002152 with value=2147483646 which is the maximum allowed value for 31 bits,
shows encoding and decoding to be OK. Tests were done with trunk rev. 137, and branch 0.8.2 rev 102

[vanh souvanlasy (vanh-souvanlasy)]

encoding and decoding of descriptor 24013 is now done properly with value such as 42949672.94 which is the maximum
allowed value for this descriptor. changes committed to trunk rev. 144