Correct return type on Batch::indices

[jbj]

Current Clang versions and GCC < 8 have ignored that the return type was wrong on this function, probably because of overeager SFINAE implementations, but it's a hard error on GCC 8.

This commit fixes compilation on Ubuntu 18.10.

[jbj]

I'm opening this PR because we'll soon upgrade lgtm.com to Ubuntu 18.10, and the upgrade will break this project and its dependencies unless the compilation error is fixed.

For the record, this is the compilation error I'm getting with GCC 8.2.0:

[2019-03-13 16:33:20] [build] In file included from /opt/src/src/importers/Importer.hpp:32,
[2019-03-13 16:33:20] [build]                  from /opt/src/src/importers/STL10MatrixImporter.hpp:28,
[2019-03-13 16:33:20] [build]                  from /opt/src/src/importers/STL10MatrixImporter.cpp:24:
[2019-03-13 16:33:20] [build] /opt/src/src/types/Batch.hpp: In member function ‘std::vector<long unsigned int> mic::types::Batch<DataType, LabelType>::indices(size_t)’:
[2019-03-13 16:33:20] [build] /opt/src/src/types/Batch.hpp:136:31: error: could not convert ‘index_->((mic::types::Batch<DataType, LabelType>*)this)->mic::types::Batch<DataType, LabelType>::sample_indices.std::vector<long unsigned int>::operator[]()’ from ‘__gnu_cxx::__alloc_traits<std::allocator<long unsigned int>, long unsigned int>::value_type’ {aka ‘long unsigned int’} to ‘std::vector<long unsigned int>’
[2019-03-13 16:33:20] [build]    return sample_indices[index_];
[2019-03-13 16:33:20] [build]                                ^