Fix crash in header parsing when trace count is parsing block_size + 1

[tasansal]

Problem

There was an edge case when the trace count was 1 more than the N*block_size for parsing the grid headers; this caused a singleton array to return.

E.g. if the block size is 100 and we have 201 traces, blocks:

1. [0, 100) = array size 100
2. [100, 200) = array size 100
3. [200, 201) = array size 1 (singleton)

In the third block, this caused unexpected indexing and subsequent concatenation errors due to us manipulating a singleton array with slicing semantics.

Solution

Ensure the new filtered array is at least 1d by using ndmin=1 in array constructor.