…al, it's now possible to specify a solver for the heatbath, different from the one used in the MD evolution (computation of the forces).

In order to do that I made the following changes to the code:

1. I added
    `int HB_solver; // solver for the HB only (optional) ` and
    `  solver_params_t HB_solver_params; // parameters of the HB solver ` to the `monomial` struct in `monomial/monomial.h`.
2. I wrote the declaration `%x HB_MSOLVER` in `read_input.l`. There I also wrote
    `
    {SPC}*HB_Solver{EQL} {
    solver_caller=YY_START;
    BEGIN(NDMSOLVER);
    }
    ` in `<NDRATMONOMIAL,NDRATCORMONOMIAL,NDCLRATMONOMIAL,NDCLRATCORMONOMIAL,NDDETRATMONOMIAL,NDCLDETRATMONOMIAL>`,
    which
    I defined the block `<HB_MSOLVER>`, analogously to `<MSOLVER>`, replacing all the occurrencies of `mnl->solver =` with `mnl->HB_solver =`.
    ** To do**: do the same with the solver_parameters
3. I added `INVALID_SOLVER` to the `SOLVER_TYPE` enum in `solver/solver_type.h`. It's value is `26`.
4. I added the macro `#define _default_HB_solver_flag 26 // this is INVALID_SOLVER (see solver/solver_types.h)` in `default_input_values.h`.
5. In `monomial/cloverdetratio_monomial.c` I changed the function `cloverdetratio_heatbath()` replacing all the occurrencies of `mnl->solver` and `mnl->solver_params` with `mnl->HB_solver` and `mnl->HB_solver_params` respectively.
6. In `monomial/monomial.c`:
    - In `add_monomial()`, I added
    `monomial_list[no_monomials].HB_solver = _default_HB_solver_flag;` in the function `add_monomial()`.
    - In `init_monomials()`, when the HB solver is invalid, i.e. `(monomial_list[i].HB_solver == INVALID_SOLVER`, I set the `HB_solver` and ` Hb_solver_params` equal to the ones used in the MD evolution. In short: if the HB solver is not specified in the input file, the solver type and its parameters are copied from what was specified for the MD. (note that the `hbfunction` is the same, but now uses the HB solver and its parameters)

`<DETMONOMIAL,CLDETMONOMIAL,CLDETRATMONOMIAL,CLDETRATRWMONOMIAL,NDRATMONOMIAL,NDRATCORMONOMIAL,NDCLRATMONOMIAL,NDCLRATCORMONOMIAL,RATMONOMIAL,RATCORMONOMIAL,CLRATMONOMIAL,CLRATCORMONOMIAL,NDDETRATMONOMIAL,NDCLDETRATMONOMIAL>`
which parses, for each monomial `mnl`, the `HB_solver_params` attributes.
I did it by copying and pasting the same block for `mnl->solver_params` and preplacing those occurrences with `mnl->HB_solver_params`.
Note that the default values don't need to be specified because the default corresponds to the solver used in the MD evolution.

…atbath()` replacing all the occurrencies of `mnl->solver` and `mnl->solver_params` with `mnl->HB_solver` and `mnl->HB_solver_params` respectively.

`HB_UseExternalInverter, HB_UseSloppyPrecision, HB_UseCompression`

`
{SPC}*HB_Solver{EQL} {
solver_caller=YY_START;
BEGIN(NDMSOLVER);
`
We don't need to be able to set the HB solver also for the ND monomial.

… HB_solver also for the DETRATIO monomial.

HB_solver specified only for the cloverdetratio and detratio monomials

- Removed extra spaces in blockS (flex syntax error)
	modified:   read_input.l

…ere's a separate lib file now

…our CI