Fix #41 libpms API is missing some features available via command-line

[vasdommes]

This PR fixes #41 by adding the following functions to include/mps/context.h:

// Set the number of threads, see "mpsolve -j" option
void mps_context_set_n_threads (mps_context * s, int n_threads);

// Restrict the search set for the roots, see "mpsolve -S" option
void mps_context_set_search_set (mps_context * s, mps_search_set set);

// Detect properties of the roots, see "mpsolve -D" option
void mps_context_set_root_properties (mps_context * s, char root_properties);

// Read the starting approximations from the given file, see "mpsolve -s" option
void mps_context_set_root_stream (mps_context * s, FILE * rtstr);