Explicit pocket definition

Added a novel option to fpocket to be run with an explicitly defined pocket. This aims for supporting detections when users aren"t happy with too large or split pockets:

-P string: (None) Binding site delimited by the user through the input. You can inidcate which amino-acids are part of the binding site you'd like to "identify" and calculate descriptors for. fpocket will run its usual alpha-sphere detection and clustering will be guided to collect all alpha spheres in contact with residues of interest. You should enter a string of residues with residue numbers, insertion codes & chain codes: 'residuenumber1:insertioncode1:chaincode1.residuenumber2:insertioncode2:chaincode2.residuenumber3:insertioncode3:chaincode3'. Insertion codes can be empty. -P 107::A.138::A.51::A.98::A.55::A.93::A for instance for part of the HSP90 binding site of 4cwr. NB: If you use an mmcif file as input, you need to use the automatically assigned residue number instead of author defined number for this to work.