Don't strip input state file in GROMACS (pre-2024)

[HubLot]

This follows the bug report in GROMACS forum here.

The bug lied in the name of the colvars restart file or (colvars input state file) which was abf_dihedral_state.dat so the line input_prefix_str.erase(input_prefix_str.rfind(".colvars.state")); crashed.
Furthermore, the function colvarmodule::setup_input():

colvars/src/colvarmodule.cpp

Lines 1346 to 1351 in 351bfd2

	std::string restart_in_name(proxy->input_prefix() + std::string(".colvars.state"));
	std::istream *input_is = &(proxy->input_stream(restart_in_name, "restart file/channel", false));
	if (!*input_is) {
	// Try without the suffix ".colvars.state"
	restart_in_name = proxy->input_prefix();
	input_is = &(proxy->input_stream(restart_in_name, "restart file/channel"));

doesn't handle the case when the suffix is .dat. (suffix .dat was introduced due to a limitation in file extension in mdrun).

So the most forward bugfix is to not strip the extension and let setup_input() takes the whole string as a filename.
The downside is the variable input_prefix_str is also called in colvarmodule::print_total_forces_errning() but according to @jhenin it's only called for code older than 2016

colvars/src/colvarmodule.cpp

Line 1535 in 351bfd2

if (restart_version_number() < 20160810) {

(it's basically a revert of commit efac865)