mdds: Update to 1.5.0

Changelog:
mdds 1.5.0

* documentation

  * moved the documentation hosting to readthedocs.io, and adjusted the build
    steps.

  * moved the API incompatibility notes from README to the rst doc.

  * added the overview section for flat_segment_tree.

* multi_type_vector

  * fixed the static get(const const_position_type& pos) method for the
    boolean_element_block, by adding specialization for it to work around the
    issue with std::vector<bool> not having the at() method.

  * fixed an issue with the const position() method not returning a valid end
    position the same way the non-const variant does.

  * added steps to traverse blocks backward from the postiion specified in the
    position hint.  This may result in improved performance in some
    situations.

  * the standard integer blocks now use fixed size integer types i.e.

    * (u)int8_t

    * (u)int16_t

    * (u)int32_t

    * (u)int64_t

  * The numeric_element_block has been renamed to double_element_block.

  * added new block type to store float element values.

* general

  * added gdb pretty printers that prints the contents of the data structures.

(ryoon)