Raise exception when too many header files are provided

[adeane-ga]

Very small PR to simply generate a clearer error message for when there are too many parameter files given to PyRate ini the lists.

• manage_headers() in pyrate/core/gamma.py is designed to read and combine header information from a list of 2 MLI.par files or a single DEM.par file.
• The iff condition statement at line 457 controls the flow: https://github.com/GeoscienceAustralia/PyRate/blob/c3054c569b4b1827b61326741277606bc47e8463/pyrate/core/gamma.py#L444-L471
• if the user accidently has more than 2 MLI.par files in their list (e.g. because of multiple polarisations), then this conditional statement will will not apply.
• I have just added am elif condition to raise an error if the list of header files is > 2.

[adeane-ga]

Just added unit test to check if manage_headers() will throw exception when input IFG list has more than two files:

https://github.com/GeoscienceAustralia/PyRate/blob/75bc55be3ad885ad7296189177028943b9283dfc/tests/test_gamma.py#L178-L187