DSN transmitting frequency ramp tables overlap

[valeriof7]

In PR #268, the following code block in include/tudat/astro/ground_stations/transmittingFrequencies.h and relative unit test has been commented out, but needs to be addressed:
"""
// If there are discontinuities
if ( startTimes_.at( i ) != endTimes_.at( i - 1 ) )
{
// If the start and end times are inconsistent throw error
if ( endTimes_.at( i - 1 ) > startTimes_.at( i ) )
{
throw std::runtime_error(
"Error when creating piecewise linear frequency interpolator: inconsistency between ramp end "
"time (" + std::to_string( double( endTimes_.at( i - 1 ) ) ) + ") and start time of the following ramp (" +
std::to_string( double( startTimes_.at( i ) ) ) + "); the end is smaller than the start time." );
}
// If there are gaps in the data save that information
else
{
invalidTimeBlocksStartTimes_.push_back( endTimes_.at( i - 1 ) );
invalidTimeBlocksEndTimes_.push_back( startTimes_.at( i ) );
}
"""

In particular, we need to properly implement the logic to deal with overlaps in the DSN transmit frequency ramp table. Possible solutions suggested by @DominicDirkx:

• check if the tables are only slightly different (same frequencies possibly referenced to a different date)
• select one of the tables and reset the time range of the other one
• give a warning
• give an exception